[release/1.4.18] 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

(cherry picked from commit 40cdcf1183)

core/state/state_object.go

@@ -137,9 +137,9 @@ func (self *StateObject) markSuicided() {
 func (c *StateObject) getTrie(db trie.Database) *trie.SecureTrie {
 	if c.trie == nil {
 		var err error
-		c.trie, err = trie.NewSecure(c.data.Root, db)
+		c.trie, err = trie.NewSecure(c.data.Root, db, 0)
 		if err != nil {
-			c.trie, _ = trie.NewSecure(common.Hash{}, db)
+			c.trie, _ = trie.NewSecure(common.Hash{}, db, 0)
 			c.setError(fmt.Errorf("can't create storage trie: %v", err))
 		}
 	}