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, 8 insertions, 5 deletions

diff --git a/trie/secure_trie.go b/trie/secure_trie.go
index 2a8b57214..4d9ebe4d3 100644
--- a/trie/secure_trie.go
+++ b/trie/secure_trie.go
@@ -49,8 +49,12 @@ type SecureTrie struct {
 // If root is the zero hash or the sha3 hash of an empty string, the
 // trie is initially empty. Otherwise, New will panic if db is nil
 // and returns MissingNodeError if the root node cannot be found.
+//
 // Accessing the trie loads nodes from db on demand.
-func NewSecure(root common.Hash, db Database) (*SecureTrie, error) {
+// Loaded nodes are kept around until their 'cache generation' expires.
+// A new cache generation is created by each call to Commit.
+// cachelimit sets the number of past cache generations to keep.
+func NewSecure(root common.Hash, db Database, cachelimit uint16) (*SecureTrie, error) {
 	if db == nil {
 		panic("NewSecure called with nil database")
 	}
@@ -58,9 +62,8 @@ func NewSecure(root common.Hash, db Database) (*SecureTrie, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &SecureTrie{
-		trie: *trie,
-	}, nil
+	trie.SetCacheLimit(cachelimit)
+	return &SecureTrie{trie: *trie}, nil
 }
 
 // Get returns the value for key stored in the trie.
@@ -191,7 +194,7 @@ func (t *SecureTrie) secKey(key []byte) []byte {
 // The caller must not hold onto the return value because it will become
 // invalid on the next call to hashKey or secKey.
 func (t *SecureTrie) hashKey(key []byte) []byte {
-	h := newHasher()
+	h := newHasher(0, 0)
 	h.sha.Reset()
 	h.sha.Write(key)
 	buf := h.sha.Sum(t.hashKeyBuf[:0])