prserv/serv: Ensure sync happens in the correct thread

The sync/commit calls are happening in the submission thread which can
race against the handler. The handler may start new transactions which
then causes the submission thread to error with "cannot start a
transaction within a transaction".

The fix is to move the calls to the correct thread.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>