Adds sindex queues to the btree store.
Drop an unneeded method.
A number of steps toward working post construction.

Adds in a sindex queue. And makes writes put things in them.
Makes post construction test much harder.

Also fixes some bugs that it revealed.
Checkpoint commit.

In theory the post construction happens in the background...
however right now it hangs due to io_backender_t destructor messing
things up.
Makes io backenders be present in btree stores.
Moves the iobackender into the btree store.

This was needed for scoping reasons but also just makes more sense
because it doesn't require making new one to make a sindex.
Make sindex queue locking safer.

Locking the sindex queue now requires that you hold the sindex block.
This is enforced by passing it as an argument. Right now there's no
actual requirement that it be the correct block... but it seems unlikely
that someone would unknowingly make this mistake.
Revert first attempt at sindex queue.
Merge branch 'jd_simple_sindex_queue' into jd_secondary_indexes