CR 199/D657 (removed commented debugfs)
Increase the sindex reference size to 4080.

Addresses D577.
Merge branch 'next' into sam_207
Made the mc_transaction_t pulse the disk_ack_signal in its destructor, sometimes fakely.
Only update post construct sindexes.

We also need a test for this.
Addresses D588.
Made ~mc_transaction_t handle the disk_ack_signal == NULL case.
Make backfilling update sindexes.

Addresses D590 and D591.
Made mc_transaction_t use its own wait_for_flush field (albeit taken from the const field in the writeback).
Removed mc_transaction_t::wait_for_flush, just used null/non-nullness of disk_ack_signal.
Removed the defunct wait_for_flush variable and config parameter.
Adds comments about fields in rget_read_t.

Addresses D629.
Removes batch size from streams... again.

I know I did this but somehow I'm dong it again.
Fixes D592.
Update a comment in .proto to reflect sindexes.
Explicitly compare to NULL.

Addresses D651.
Made disk_ack_signals be a sync_callback_t.
Made sync_callback be passed directly to the writeback instead of holding up transaction destruction.
A large number of fixed from CodeReview #306
Made btree_slice_t::create wait for its disk_ack_signal.
Adds readme explaining docs system
Merge branch 'rql_docs_conversion' of github.com:rethinkdb/rethinkdb into rql_docs_conversion