trie, core/state: improve memory usage and performance (#3135)

* trie: store nodes as pointers

This avoids memory copies when unwrapping node interface values.

name      old time/op  new time/op  delta
Get        388ns ± 8%   215ns ± 2%  -44.56%  (p=0.000 n=15+15)
GetDB      363ns ± 3%   202ns ± 2%  -44.21%  (p=0.000 n=15+15)
UpdateBE  1.57µs ± 2%  1.29µs ± 3%  -17.80%  (p=0.000 n=13+15)
UpdateLE  1.92µs ± 2%  1.61µs ± 2%  -16.25%  (p=0.000 n=14+14)
HashBE    2.16µs ± 6%  2.18µs ± 6%     ~     (p=0.436 n=15+15)
HashLE    7.43µs ± 3%  7.21µs ± 3%   -2.96%  (p=0.000 n=15+13)

* trie: close temporary databases in GetDB benchmark

* trie: don't keep []byte from DB load around

Nodes decoded from a DB load kept hashes and values as sub-slices of
the DB value. This can be a problem because loading from leveldb often
returns []byte with a cap that's larger than necessary, increasing
memory usage.

* trie: unload old cached nodes

* trie, core/state: use cache unloading for account trie

* trie: use explicit private flags (fixes Go 1.5 reflection issue).

* trie: fixup cachegen overflow at request of nick

* core/state: rename journal size constant

1 files changed, 7 insertions, 4 deletions

diff --git a/core/state/statedb.go b/core/state/statedb.go
index ec9e9392f..dcb897628 100644
--- a/core/state/statedb.go
+++ b/core/state/statedb.go
@@ -41,7 +41,10 @@ var StartingNonce uint64
 const (
 	// Number of past tries to keep. The arbitrarily chosen value here
 	// is max uncle depth + 1.
-	maxTrieCacheLength = 8
+	maxPastTries = 8
+
+	// Trie cache generation limit.
+	maxTrieCacheGen = 100
 
 	// Number of codehash->size associations to keep.
 	codeSizeCacheSize = 100000
@@ -86,7 +89,7 @@ type StateDB struct {
 
 // Create a new state from a given trie
 func New(root common.Hash, db ethdb.Database) (*StateDB, error) {
-	tr, err := trie.NewSecure(root, db)
+	tr, err := trie.NewSecure(root, db, maxTrieCacheGen)
 	if err != nil {
 		return nil, err
 	}
@@ -155,14 +158,14 @@ func (self *StateDB) openTrie(root common.Hash) (*trie.SecureTrie, error) {
 			return &tr, nil
 		}
 	}
-	return trie.NewSecure(root, self.db)
+	return trie.NewSecure(root, self.db, maxTrieCacheGen)
 }
 
 func (self *StateDB) pushTrie(t *trie.SecureTrie) {
 	self.lock.Lock()
 	defer self.lock.Unlock()
 
-	if len(self.pastTries) >= maxTrieCacheLength {
+	if len(self.pastTries) >= maxPastTries {
 		copy(self.pastTries, self.pastTries[1:])
 		self.pastTries[len(self.pastTries)-1] = t
 	} else {