Implement a batching rescan manager.

Recent btcd versions only allow one rescan to run at any given time
per websocket client.  To better handle this, a new set of goroutines
are started by the account manager which batch and serialize rescan
jobs.

If no rescans are currently running, a new rescan starts.  If a rescan
is already being processed, the request is queued and runs after the
current rescan finishes.  For any additional incoming requests before
the current rescan finishes, the requests are merged with the
currently-waiting request so both can be handled with a single rescan.

This change also prepares for rescan progress notifications from btcd,
but are still unhandled until the necessary details for
partially-synced addresses are added to the wallet file format.