fix more syncing issues

[Westlad]

This fixes a number of corner-case syncing issues:

1. When optimist synchronises, it's possible that new transactions could come in. These were ignored. To fix this, we subscribe to events from the L1 block one after syncing ends, rather than from the current block, which may be ahead by some distance.
2. offchain transactions are now queued along with onchain ones. This gives a consistent way of managing them during synchronisation (and more generally).
3. When syncing, the block and transaction queues are paused. They will still accept new transactions but these are not processed until syncing is finished.