Support set/delete of timers by ID in Flink runner

This is a prerequisite for full support of BEAM-27, and related to BEAM-35.

My current understanding is that this might be actually removed from Flink itself, and needs implementation in terms of state. It might be reasonable to just remove deletion from Beam; it is just a nice-to-have for user experience IMO, and perhaps a performance help for certain runners.

(Since I am filing individual tickets for each runner, generalized discussion should probably be on the dev list directly on or BEAM-35)