WorkManager should allow Work execution to be delayed after the transaction commit

Many Work instances will need to read data written by the main thread that created the work instance. In order for the work instance to see this data, the main thread must have committed its transaction.

This means that submitting Work to the WorkManager must, in most cases, be delayed until after transaction commit.

(We should leave the possibility though to execute the Work instance as soon as possible without waiting for transaction commit, in the case where the work instance does not care about it.)