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, 2 insertions, 2 deletions

diff --git a/trie/secure_trie_test.go b/trie/secure_trie_test.go
index 3171b8c31..159640fda 100644
--- a/trie/secure_trie_test.go
+++ b/trie/secure_trie_test.go
@@ -29,7 +29,7 @@ import (
 
 func newEmptySecure() *SecureTrie {
 	db, _ := ethdb.NewMemDatabase()
-	trie, _ := NewSecure(common.Hash{}, db)
+	trie, _ := NewSecure(common.Hash{}, db, 0)
 	return trie
 }
 
@@ -37,7 +37,7 @@ func newEmptySecure() *SecureTrie {
 func makeTestSecureTrie() (ethdb.Database, *SecureTrie, map[string][]byte) {
 	// Create an empty trie
 	db, _ := ethdb.NewMemDatabase()
-	trie, _ := NewSecure(common.Hash{}, db)
+	trie, _ := NewSecure(common.Hash{}, db, 0)
 
 	// Fill it with some arbitrary data
 	content := make(map[string][]byte)