Store new StackerDB chunks from event observer

[hstove]

The API's event receiver doesn't handle the /stackerdb_chunks path, so those events are not stored in the DB's event_observer_requests table. This would be handy for acting as a starting point to analyze StackerDB data, like when/which signers are responding to blocks.

For now, I think it would be helpful to just add a handler for the POST /stackerdb_chunks path that dumps the payload into event_observer_requests - no other action needed.

Note that this path is only hit if the Stacks node has stacker = true