[chainstate] defer trie root hash calculation until no more writes will occur on the trie

[jcnelson]

Right now, writing nodes to a trie in the MARF consumes in aggregate a lot of time, because each time a leaf is inserted, the trie root hash is recalculated. This is unnecessary; the root hash only needs to be calculated once we're done writing leaf nodes. To implement this, the MARF needs a seal() operation that (a) calculates the trie root hash and the MARF root hash in the uncommitted state and stores it there, and (b) prevents subsequent writes from happening on the uncommitted state. Reads and transaction abort/commit will continue to be permitted.