Add a method to get the tree size of the local database.

This is to be able to resume from a partially replicated database after a restart.
This method is meant to be "what tree size could we be serving". Right now, the ```FileDB``` version does a CHECK to make sure it isn't sparse, so sparseness isn't well supported, and you'll right that I'll need it (as you say, even though I prioritize the oldest entries, there might well be gaps, at least momentarily).

But this commit seemed useful to "fix" your ```signer``` branch, even though it doesn't do everything I need for replication.

For the replication, I'm not sure if I'll add a method to get the sparse entries, or if I won't just resume from the tree size, in case of a crash/restart... Not sure that getting the sparse entries is worth the effort?