The disk syncer now maintains its own countdown timer, creating a new
timer only when necessary (when there is no timer running, and
something is scheduled to be written). When the timer expires, the
select loop begins selecting on a grab of the account manager's binary
semaphore, and if read, performs the sync and nils the select channel
to prevent a future grab until a new timer has expired.
Tested with a race-enabled build on Windows. No lockups or races
related to the disk syncing experienced with constant client requests
and incoming btcd notifications, and scheduled writes run as expected
once the countdown timer expires, locking out all server request and
notifiation handling.
This change removes the three separate mutexes which used to lock an
account's wallet, tx store, and utxo store. Accounts no longer
contain any locking mechanism and rely on go's other synchronization
constructs (goroutines and channels) for correct access.
All accounts are now managed as a collection through the new
AccountManager, rather than the old AccountStore. AccountManager runs
as its own goroutine to provide access to accounts.
RPC requests are now queued for handling, being denied if the queue
buffer is exhausted. Notifications are also queued (instead of being
sent from their own goroutine after being received, in which order is
undefined), however, notifications are never dropped and will
potentially grow a queue of infinite size if unhandled.
