Rip out address pools

[jrick]

These are a hack on top of a waddrmgr that was unable to derive and record addresses beyond the next to use address. They should not belong in the code any longer. In addition, they seem to be non-atomic and rely on clean database shutdown in order to update waddrmr with their latest state.

I don't expect any performance penalty for removing them either, since database update transactions are already always required to return a new address from them, in the case that more addresses must be derived.

Removing address pools should also bring the code closer to btcwallet making any future syncs slightly less awful.