Improve websocket transaction notifications.

This change improves the mechanism by which btcd notifies a websocket
client of transaction relating to watched address and unspent outputs
in the following ways:

1. The old processedtx notification has been replaced with the new
   recvtx notification.  This notification, rather than parsing out
   details used by wallet clients, sends the serialized transaction
   (as hexadecimal) and any block details (if mined) if any transaction
   output sends to one of the websocket client's watched addresses.

2. The old txspent notification has been replaced with the new
   redeemingtx notification.  This notification, rather than parsing
   out details used by wallet clients, sends the serialized transaction
   (as hexadecimal) and any block details (if mined) if any transaction
   input spends a watched output.

3. When processing notifications for transaction outputs, if any output
   spends to a client's watched address, a corresponding spent request
   is automatically registered.

4. Transaction notifications originating from mempool now include both
   transaction inputs and outputs, rather than only processing

5. When processing notifications for transaction inputs, a client's
   output spent request is only removed if the transaction being
   processed has also been mined into a block.  In combination with the
   4th change, this results in two redeemingtx notifications for
   transactions which first appear in mempool and are subsequently mined
   into a block.