Implement graceful cancellation to terminate PostgreSQL queries

[Weixing-Zhang]

Currently, interrupting martin or martin-cp (e.g., via Ctrl+C) stops the client process, but it does not terminate the corresponding active queries on the database side. Ideally, a client-side cancellation should propagate to PostgreSQL to stop and clean up the backend processes.

Reference:
This is similar to the behavior reported here: #6

cc: @nyurik

[Weixing-Zhang]

I'm planning to take a stab at this issue and will post updates here. That said, anyone else is more than welcome to tackle it as well.