DB-backed UtxoSet

[fungibit]

For tracked spending, should support using an alternative implementation of UtxoSet which stores the data in a DB (e.g. mongodb).

This will allow running with track_spending=True on machines with moderate amount of free RAM.

[fungibit]

A suggestion by Rusty Russel is to also support "checkpointing": e.g. keep a copy of the utxoset after block N.

This can be useful if some program always needs to start processing the blockchain at block N. They can checkpoint it once, and then start from there on each run.

If we implement a mongodb-collection-backed utxoset, checkpointing can be achieved by cloning the collection after a certain block.