Change timer id type from string to int

Timer service currently uses UUID string value as timer id. While it's guaranteed to be unique, it is not efficient in terms of generating UUID, storing it in database, indexing and querying it. Especially in database timer store, consider using int (identity, or generated sequence in database table column) as the type of timer id.

This should not impact file store persistent timers, which will continue using UUID as timer id.

The updated timer service should still be able to work with the old timer table. Existing timers in the timer table, whose id column type is varchar, should still be able to work.