FileStore: replace op_queue_throttle with op_queue_reserve_throttle

FileStore: replace op_queue_throttle with op_queue_reserve_throttle

Previously, queue_op would call op_queue_throttle while holding the
journal_lock.  op_queue_throttle, however, can sleep.

We fix the problem by:
1) Factor build_op out of queue_op
2) op_queue_throttle is now op_queue_reserve_throttle and takes an op as
an argument.  op_queue_reserve_throttle can be called before the journal
lock is taken.  This also avoids the race between calling throttle and
incrementing op_queue_bytes and op_queue_len.
3) queue_op now takes the op generated using build_op as an argument.
4) _journaled_ahead no longer needs to call throttle as
queue_transactions has already reserved space.

Signed-off-by: Samuel Just <samuel.just@dreamhost.com>