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

trie/trie_test.go

@@ -460,8 +460,7 @@ const benchElemCount = 20000
 func benchGet(b *testing.B, commit bool) {
 	trie := new(Trie)
 	if commit {
-		dir, tmpdb := tempDB()
-		defer os.RemoveAll(dir)
+		_, tmpdb := tempDB()
 		trie, _ = New(common.Hash{}, tmpdb)
 	}
 	k := make([]byte, 32)
@@ -478,6 +477,13 @@ func benchGet(b *testing.B, commit bool) {
 	for i := 0; i < b.N; i++ {
 		trie.Get(k)
 	}
+	b.StopTimer()
+
+	if commit {
+		ldb := trie.db.(*ethdb.LDBDatabase)
+		ldb.Close()
+		os.RemoveAll(ldb.Path())
+	}
 }
 
 func benchUpdate(b *testing.B, e binary.ByteOrder) *Trie {