Added per disk PUT timing monitoring support.

Fixes bug 1104708

There could be severe performance drop for swift is one disk of one
storage node is problematic due to the tragic state of async disk I/O.

This patch provided PUT timing per kB transfered (ms/kB) monitoring
support for each non-zero-byte request of each disk and report to
statsD for alert.
-adding "object-server.PUT.<device>.timing" metrics for object-server.

DocImpact.

Change-Id: Ie94bddad28e8be52e71683bf6c9db988664abe47
Merge "Added per disk PUT timing monitoring support."