Retain order when processing client notifications.

This change modifies the RPC server's notifiation manager from a
struct with requests, protected by a mutux, to two goroutines.  The
first maintains a queue of all notifications and control requests
(registering/unregistering notifications), while the second reads from
the queue and processes notifications and requests one at a time.

Previously, to prevent slowing down block and mempool processing, each
notification would be handled by spawning a new goroutine.  This lead
to cases where notifications would end up being sent to clients in a
different order than they were created.  Adding a queue keeps the
order of notifications originating from the same goroutine, while also
not slowing down processing while waiting for notifications to be
processed and sent.

ok @davecgh