Integrates the blockchain v2 reactor with state sync, fixes#4765. This mostly involves deferring fast syncing until after state sync completes. I tried a few different approaches, this was the least effort:
* `Reactor.events` is `nil` if no fast sync is in progress, in which case events are not dispatched - most importantly `AddPeer`.
* Accept status messages from unknown peers in the scheduler and register them as ready. On fast sync startup, broadcast status requests to all existing peers.
* When switching from state sync, first send a `bcResetState` message to the processor and scheduler to update their states - most importantly the initial block height.
* When fast sync completes, shut down event loop, scheduler and processor, and set `events` channel to `nil`.