In this commit, we extend the client's filtering process to also look at
the set of currently watched transactions. The logic to watch for
transaction hashes was previously there, but it was not used to filter
against incoming transactions.
In the previous commit, we modified our BitcoindClient struct to use a
ConcurrentQueue struct to handle its notifications to the caller. Before
this, the BitcoindClient had a goroutine that would handle these
notifications in the background and detect when a OnBlockConnected
notification was received in order to update the best block. Due to this
logic being removed, we now keep track of the best block througout the
struct as a whole.
