Graceful teardown

[ricellis]

Following on from point 1 of issue #65 we could do more to teardown gracefully. For example we should complete writing inflight batches before exiting. This is also related to #60 in that there should be internal teardown mechanisms that can be called from either a CLI style cancel like SIGTERM or by an API invoked cancel once #60 is done.

• couchbackup/couchrestore version(s) that are affected by this issue.

All

• Node.js version.

Any

• A sample of the code or command line statement that demonstrates the issue.