monitor: refactor/rewrite to use new async API

monitor: refactor/rewrite to use new async API

In order to migrate to the upcoming Danga::Socket-based
HTTP API, we'll first refactor monitor to use the new API
(but preserve LWP usage behind-the-scenes).

DEBUG=1 users will see the elapsed time for all device refreshes
each time monitor runs.

While we're at it, also guard against race conditions on the
PUT/GET test by double-checking on failure.  (A long-standing
TODO item)

also squashed the following commit:

  use conn_timeout in monitor, node_timeout in other workers

  This matches the behavior in MogileFS:Server 2.65.

  It makes sense to use a different, lower timeout in monitor to
  quickly detect overloaded nodes and avoid propagating their
  liveness for a monitoring period.

  It also makes sense to use a higher value for node_timeout in
  other workers since other actions are less fault-tolerant.

  For example, a timed-out size check in create_close may cause a
  client to eventually reupload the file, creating even more load
  on the cluster.