- don't use fiber_yield_timeout, it's too slow; use a single
idle timer and invoke the fiber which hasn't been invoked for longest
period so that it can time out

- chain up the running fibers in a pool so that coro_transfer()
goes to the next fiber, not to the sched fiber, most of the time

This together improves cbus speed and reduces CPU spent in
coro_transfer().

eio_req must be freed from req->destroy instead of req->finish.

Ideally, the batch should be written to WAL in a single write.

Link all ready fibers into a call chain, so that
fiber_yield() directly involves the next ready fiber,
eventually returning to the fiber initiating the schedule.

This slashes the amount of coro_transfer() in fiber_schedule_list()
in half.

Fix a possible race condition in checks for pool->fetch_output
event.

Limit the iproto pipe max input to ensure smooth operation
when there are many messages.

 * extra arguments silently ignored
 * :get and :put interpret nil timeout as infinity

Since in a shared pool we can deadlock if we run out of fibers, shift the
responsibility of pipeline congestion control from fiber pool to iproto
thread, and make the pool size effectively unlimited.

- optimize fiber pool yield performance even further under busy
workload scenario.

- remove reciprocal message fetch
- use a shared fiber pool
- temporarily disable lock statistics
- use distinct mutexes for every cord

Using Mike Pall's advanced readline patch,
http://smbolton.com/lua.html#readline

Don't start the wakeup event, it's only used internally with custom
events.

This patch gives up to 200 000 extra rps in wal_mode=none on
my laptop (1 450 000 vs. 1 250 000, nosqlbench 10x1000 batch workload)

I never remember using it, must be some ancient artifact.