Audit usages of epoch in sharding code

In several places in sharding we use a change of epoch to indicate that a collection had been dropped and recreated. After the refine shard key project however, this logic is no longer valid, since refining a shard key can cause an epoch change. We should audit and fix the places where epoch is used instead of collection UUID to check for collection drops and recreates.

Specific examples include the MigrationDestinationManager and the CollectionShardingRuntime.