Added commands to support notifyallnewtxs

Sending NotifyAllNewTXsCmd will register websocket client to receive
notifications on all new transactions.

Once registered the client will receive either AllTxNtfn or
AllVerboseTxNtfn based on the required verbose field in the
NotifyAllNewTXsCmd.
Changed NewNotifyAllNewTXsCmd to optionally accept verbose arg
Added explicit json key names to new tx notifications

Also added check for nil ID on AllVerboseTxNtfn.UnmarshalJSON to make it
consistent with other notification handling.