This removes last set of references to IndexManager and integrates the
IndexSubscriber into the chain. All indexes have now been updated to
subscribe to the index subscriber for updates.
Indexes may now optionally provide their own implementation for
dropping the index, with a fallback to simply removing the index
bucket and metadata if not implemented.
Using an interface and dynamically dispatching to the correct drop
implementation also allowed removing a special case for deletion of
the transaction index from the common drop code.
