[chainstate] implement cache strategies for nodes in tries that are persisted to disk

[jcnelson]

Once a trie is stored to the MARF, it will never be modified. We should be able to fearlessly cache any/all of its nodes in RAM. The trie storage system should be updated to support a variety of caching strategies, which the user can choose in the config file (i.e. to trade RAM for speed). At a minimum, we should support the following:

• No-op (the default; what happens now)
• Everything (keep all nodes in RAM -- useful for miners)
• Node256 (keep TrieNode256's in RAM, but nothing else)

I've tried all of the above. Node256 offers decent performance without eating your RAM, for example.

[gregorycoppola]

Related: This issue is the line of work suggested in #3014, which suggests looking for ways to cache the MARF in RAM to improve performance.