Handle `SIGTERM` messages to shut down gracefully

Summary

As brought up in this slack conversation, we have an opportunity to handle SIGTERM messages gracefully.

Proposed benefits

This could improve SDK-based taps' functionality when being told to shut down gracefully.

Proposal details

• For any sorted incremental streams: we can flush the current stream and cancel syncing any other records.
• For non-sorted streams and/or streams which do not support incremental refresh: we may choose to continue syncing until the stream sync completes (or until SIGKILL forces shutdown).
• Avoid starting any other streams once the running streams are handled, as specified above.

Best reasons not to build

No downsides, no reason not to implement.