-------------
- Added `SignalBus.flushmany` method and `flushmany` CLI command.
- Added `wait` optional argument to the `SignalBus.flush` method and
the `flush` CLI command.
- Added optional `signalbus_autoflush` attribute to signal models,
which defaults to `True`.
- Fixed a bug caused by not clearing the set of signals added to the
session on rollback.
- Change `flush` and `flushmany` to obtain row lock before sending the
message, thus avoiding some rare concurrency issues.
- Improved docs