Remove several RSIs that were just questions.
Fixes drop_sindex as per RSIs.
Fix drop_all_sindexes.

The changes are aprroximately identical to those made in drop_sindexes.
Similar enough that I moved a lot of the logic in to a common function.
real_superblock_t now takes an rvalue ref.

Previously it would call the swap method on the buf_lock it was passed.
No need to swap when you have move semantics.
Add a comment explaining an RSI.
Addresses several RSIs.

We release the superblock a little bit earlier than before in
store_t::store_t.

Comment about how txn will be removed in rdb_read_visitor_t.

Remove some bs with get_event_log().
sindex_block doesn't exist when SLICE_ALT == 1

Removes an RSI to this effect.
Removing several RSIs.