Handle graceful shutdown

[zone117x]

Chainhook is ignoring SIGTERM signals and requires forced shutdown.

With fast nakamoto blocks, this is very likely to cause de-syncs with the stacks-node because in-process event ingestion is force killed before event http POST can be responded to, and before sqlite/rocksdb/redis data can be flushed.

[rafaelcr]

Although this is no longer relevant for Stacks, we should do it for Bitcoin's zeromq interface