Handles SIGINT (Ctr-C) and gracefully stops an inflight request.

[yfuruyama]

Currently spanner-cli doesn't handle SIGINT, so if it receives SIGINT in response to the Ctrl-C, the CLI is killed suddenly and inflight request is not canceled properly.

This PR handles the SIGINT properly for both interactive mode and batch mode. This PR also removes the Session.ctx field, which was not used effectively at this moment.

More background: #133

Example

Interactive mode

spanner> SELECT COUNT(*) FROM t1;
^CERROR: spanner: code = "Canceled", desc = "context canceled"

Batch mode

spanner-cli -p <PROJECT> -i <INSTANCE> -d <DATABASE> -e 'SELECT COUNT(*) FROM t1;'
^CERROR: spanner: code = "Canceled", desc = "context canceled

[yfuruyama]

@apstndb Could you take a look at this PR when you have a chance? (cc: @nielm)

[apstndb]

LGTM

[yfuruyama]

Thanks @apstndb for the review!