orderwatch,core,blockwatch: Improve Order Event subscription stability

[fabioberger]

This PR ensures that Mesh's OrderWatcher is in complete lock-step about what the latest block it knows about is, and therefore also the block at which validations/re-validations are to be conducted. While these eth_call's are in progress, no further block events are processed, so that we don't introduce any race conditions. This eliminates several bugs:

• Incoming orders being validated at an outdated block height and us missing block events processed while the validation was ongoing. (core: Validation of new orders occurring at latest block #590)
• Blocks being discovered and stored to the DB but not processed before Mesh gets shut down. Upon re-booting it assumes all stored blocks have been processed. We now only store blocks once they've been fully processed. (blockwatch,orderwatch: Store blocks in DB only _after_ processed by OrderWatcher #588)

Additionally, it:

• Batch emits ADDED order events (orderwatch: Batch order events when orders added to Mesh #559)
• Properly passes down the top-level context to every order validation eth_call request.
• Ensures that at least one block was processed before processing incoming orders (RPC/P2P), so we have an updated block height at which to process them.

[albrow]

Mostly looks pretty good. Much fewer suggestions this time and most of them are minor. I also messaged you separately about test coverage for the Cleanup method.

[albrow]

Also need to address merge conflicts now that #580 is merged.