swarm: network rewrite merge

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+## PSS tests failures explanation
+
+This document aims to explain the changes in https://github.com/ethersphere/go-ethereum/pull/126 and how those changes affect the pss_test.go TestNetwork tests.
+
+### Problem
+
+When running the TestNetwork test, execution sometimes:
+
+* deadlocks
+* panics
+* failures with wrong result, such as:
+
+```
+$ go test -v ./swarm/pss -cpu 4 -run TestNetwork
+```
+
+```
+--- FAIL: TestNetwork (68.13s)
+    --- FAIL: TestNetwork/3/10/4/sim (68.13s)
+        pss_test.go:697: 7 of 10 messages received
+        pss_test.go:700: 3 messages were not received
+FAIL
+```
+
+Moreover execution almost always deadlocks with `sim` adapter, and `sock` adapter (when buffer is low), but is mostly stable with `exec` and `tcp` adapters.
+
+### Findings and Fixes
+
+#### 1. Addressing panics
+
+Panics were caused due to concurrent map read/writes and unsynchronised access to shared memory by multiple goroutines. This is visible when running the test with the `-race` flag.
+
+```
+go test -race -v ./swarm/pss -cpu 4 -run TestNetwork
+
+  1 ==================
+  2 WARNING: DATA RACE
+  3 Read at 0x00c424d456a0 by goroutine 1089:
+  4   github.com/ethereum/go-ethereum/swarm/pss.(*Pss).forward.func1()
+  5       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/swarm/pss/pss.go:654 +0x44f
+  6   github.com/ethereum/go-ethereum/swarm/network.(*Kademlia).eachConn.func1()
+  7       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/swarm/network/kademlia.go:350 +0xc9
+  8   github.com/ethereum/go-ethereum/pot.(*Pot).eachNeighbour.func1()
+  9       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/pot/pot.go:599 +0x59
+  ...
+
+ 28
+ 29 Previous write at 0x00c424d456a0 by goroutine 829:
+ 30   github.com/ethereum/go-ethereum/swarm/pss.(*Pss).Run()
+ 31       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/swarm/pss/pss.go:192 +0x16a
+ 32   github.com/ethereum/go-ethereum/swarm/pss.(*Pss).Run-fm()
+ 33       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/swarm/pss/pss.go:185 +0x63
+ 34   github.com/ethereum/go-ethereum/p2p.(*Peer).startProtocols.func1()
+ 35       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/p2p/peer.go:347 +0x8b
+ ...
+```
+
+##### Current solution
+
+Adding a mutex around all shared data.
+
+#### 2. Failures with wrong result
+
+The validation phase of the TestNetwork test is done using an RPC subscription:
+
+```
+    ...
+	triggerChecks := func(trigger chan discover.NodeID, id discover.NodeID, rpcclient *rpc.Client) error {
+		msgC := make(chan APIMsg)
+		ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+		defer cancel()
+		sub, err := rpcclient.Subscribe(ctx, "pss", msgC, "receive", hextopic)
+		...
+```
+
+By design the RPC uses a subscription buffer with a max length. When this length is reached, the subscription is dropped. The current config value is not suitable for stress tests.
+
+##### Current solution
+
+Increase the max length of the RPC subscription buffer.
+
+```
+const (
+	// Subscriptions are removed when the subscriber cannot keep up.
+	//
+	// This can be worked around by supplying a channel with sufficiently sized buffer,
+	// but this can be inconvenient and hard to explain in the docs. Another issue with
+	// buffered channels is that the buffer is static even though it might not be needed
+	// most of the time.
+	//
+	// The approach taken here is to maintain a per-subscription linked list buffer
+	// shrinks on demand. If the buffer reaches the size below, the subscription is
+	// dropped.
+	maxClientSubscriptionBuffer = 20000
+)
+```
+
+#### 3. Deadlocks
+
+Deadlocks are triggered when using:
+* `sim` adapter - synchronous, unbuffered channel
+* `sock` adapter - asynchronous, buffered channel (when using a 1K buffer)
+
+No deadlocks were triggered when using:
+* `tcp` adapter - asynchronous, buffered channel
+* `exec` adapter - asynchronous, buffered channel
+
+Ultimately the deadlocks happen due to blocking `pp.Send()` call at:
+
+ 		 // attempt to send the message
+  		err := pp.Send(msg)
+  		if err != nil {
+  			log.Debug(fmt.Sprintf("%v: failed forwarding: %v", sendMsg, err))
+  			return true
+  		}
+
+ `p2p` request handling is synchronous (as discussed at https://github.com/ethersphere/go-ethereum/issues/130), `pss` is also synchronous, therefore if two nodes happen to be processing a request, while at the same time waiting for response on `pp.Send(msg)`, deadlock occurs.
+ 
+ `pp.Send(msg)` is only blocking when the underlying adapter is blocking (read `sim` or `sock`) or the buffer of the connection is full.
+ 
+##### Current solution
+
+Make no assumption on the undelying connection, and call `pp.Send` asynchronously in a go-routine.
+
+Alternatively, get rid of the `sim` and `sock` adapters, and use `tcp` adapter for testing.