Tear serializer and cache out of disk back queue.

You can now construct a disk backed queue using a cache you get from
somewhere else. This will help with using them for the sindex queue.
Adds in a superblock for the disk_backed_queue_t.

This allows them to be recreated after a shutdown.
Remove an acquire_sindex_block_for_write method.

We had 2 and there was no reason for it. I also removed the long winded
and incorrect explanation for why we needed both.
Start of post construction accounting.

Correctly sets the post construction flag and adds a away to acquire
only those superblocks that are not post constructing.
Post construction queueing works in theory.

Meaning that when a sindex is post constructing and someone tries to do
a write that modification will be stored in a queue and popped off when
the sindex is post constructed. Right now we don't have the popping off
part written. Just the queueing.