Parallel object auditor

We are soon going to put servers with a high ratio of disk to CPU
into production as object servers.  One of our concerns with this
configuration is that the object auditor would take too long to
complete its audit cycle.  Therefore we decided to parallelise
the auditor.

The auditor already uses fork(), so we decided to use the parallel
model from the replicator.  Concurrency is set by the concurrency
parameter in the auditor stanza, which sets the number of parallel
checksum auditors.  The actual number of parallel auditing processes
is concurrency + 1 if zero_byte_fps is non-zero.

Only one ZBF process is forked, and a new ZBF process is forked as
soon as the current ZBF process finishes.  Thus the last process
running will always be a ZBF process.

Both forever and once modes are parallelised.

Each checksum auditor process submits a nested dictionary with keys
{'object_auditor_stats_ALL': {'diskn': {..}}} to dump_recon_cache
so that the object_auditor_stats_ALL dict in recon cache consists
of individual sub-dicts for each of the object disks on the server.
The recon cache is no different to before when the checksum auditor
is run in serial mode.  When swift-recon is run, it sums the stats
for the individual disks.

DocImpact

Change-Id: I0ce3db57a43e482d4be351cc522fc9060af6e2d3
Merge "Parallel object auditor"