Use threadpools in the object server for performance.

Without a (per-disk) threadpool, requests to a slow disk would affect
all clients by blocking the entire eventlet reactor on
read/write/etc. The slower the disk, the worse the performance. On an
object server, you frequently have at least one slow disk due to
auditing and replication activity sucking up all the available IO. By
kicking those blocking calls out to a separate OS thread, we let the
eventlet reactor make progress in other greenthreads, and by having a
per-disk pool, we ensure that one slow disk can't suck up all the
resources of an entire object server.

There were a few blocking calls that were done with eventlet.tpool,
but that's a fixed-size global threadpool, so I moved them to the
per-disk threadpools. If the object server is configured not to use
per-disk threadpools, (i.e. threads_per_disk = 0, which is the
default), those call sites will still ultimately end up using
eventlet.tpool.execute. You won't end up blocking a whole object
server while waiting for a huge fsync.

If you decide not to use threadpools, the only extra overhead should
be a few extra Python function calls here and there. This is
accomplished by setting threads_per_disk = 0 in the config.

blueprint concurrent-disk-io
Change-Id: I490f8753d926fdcee3a0c65c5aaf715bc2b7c290
Merge "Use threadpools in the object server for performance."