feat(gapic-common): Support timeout-based polling

[aandreassa]

Start supporting timeouts in the gapic polling harness.

It removes the need for clients to keep track of their own time variables, as it is done here in bigtable.

Note: timeout deadline is calculated based on when the method wait is called. Late initialization can cause off-by-one errors.

Next PR integrates this change for testing in bigtable.