trie: rename TrieSync to Sync and improve hexToKeybytes (#16804)

This removes a golint warning: type name will be used as trie.TrieSync by
other packages, and that stutters; consider calling this Sync.

In hexToKeybytes len(hex) is even and (even+1)/2 == even/2, remove the +1.

trie/encoding.go

@@ -83,7 +83,7 @@ func hexToKeybytes(hex []byte) []byte {
 	if len(hex)&1 != 0 {
 		panic("can't convert hex key of odd length")
 	}
-	key := make([]byte, (len(hex)+1)/2)
+	key := make([]byte, len(hex)/2)
 	decodeNibbles(hex, key)
 	return key
 }