DETACH PARTITION: hold locks on indexes until end of transaction

DETACH PARTITION: hold locks on indexes until end of transaction

When a partition is detached from its parent, we acquire locks on all
attached indexes to also detach them ... but we release those locks
immediately.  This is a violation of the policy of keeping locks on user
objects to the end of the transaction.  Bug introduced in 8b08f7d4820f.

It's unclear that there are any ill effects possible, but it's clearly
wrong nonetheless.  It's likely that bad behavior *is* possible, but
mostly because the relation that the index is for is only locked with
AccessShareLock, which is an older bug that shall be fixed separately.

While touching that line of code, close the index opened with
index_open() using index_close() instead of relation_close().
No difference in practice, but let's be consistent.

Unearthed by Robert Haas.

Discussion: https://postgr.es/m/CA+TgmoYruJQ+2qnFLtF1xQtr71pdwgfxy3Ziy-TxV28M6pEmyA@mail.gmail.com