This commit fixes a possible race condition wherein a call to
FilterBlock after a call to UpdateFilter would result in the call to
FilterBlock not yet using the updated filter. We fix this by ensuring
the internal chain filter is updated by the time the call to
FilterBlock returns.
This commit adds the first concrete implementation of the
chainview.FilteredChainView interface. The implementation of this
interface, BtcdFilteredChainView is backed by a web sockets connection
to an active btcd instance.