ygnmi.Watch should handle implicit deletions via time expiry in SAMPLE mode

[wenovus]

Spec: https://github.com/openconfig/reference/pull/201/files

Currently ygnmi.Watch maintains state regardless of STREAM subscription mode, which would represent the wrong value. This is not determinable via timestamps due to having a single timestamp for the entire query object rather than per-leaf timestamps.

I think handling this within ygnmi is the more intuitive option. This is possible via an internal expiry timer. The easier solution of using sync_response is not available because they're not mandatory per the spec.