index_destroy() must grab exclusive access to the parent table

of the index it wants to destroy.  This ensures that no other backend is
actively scanning or updating that index.  Getting exclusive access on
the index alone is NOT sufficient, because the executor is rather
cavalier about getting locks on indexes --- see ExecOpenIndices().
It might be better to grab index locks in the executor, but I'm not
sure the extra lockmanager traffic is really worth it just to make
index_destroy cleaner.