PeerBlockSyncManager: check block existence in separate chainstate call

[ImplOfAnImpl]

PeerBlockSyncManager now checks whether the block already exists via a separate chainstate subsystem call, which should reduce pressure on the chainstate during IBD.

I also added a few tests:

1. in p2p, to ensure that PeerBlockSyncManager always checks for block existence right before calling process_block.
2. a test for EventsController in utils, to ensure that events are handled in the same order as they are broadcast.