Windows: Fairness for background operations

Windows: Fairness for background operations

The background daemon worker pool is responsible for processing
background Store and Fetch operations.  With the SMB interface
primary store and fetch operations are performed in the SMB worker
thread which makes sense since those operations must be synchronous
to the incoming request.

With the AFS redirector interface almost all of the work is performed
by the background daemon worker pool.  It is therefore critical that
the workers not get stuck in a state that starves applications.
For example, copy of a file that is larger than the cache to \\AFS
will result in a background store request for each chunk size of
the file.  If each worker thread grabs one to process, only one will
make progress and the rest will block.  If a cleanup operation
(aka handle close) occurs the entire file will be flushed to the
server synchronously in the redirector worker thread.  That thread
will cause of the background daemon threads to block.

Any subsequent fetch data requests that get queued behind the list
of stores will in turn block until they clear.  This behavior is not
fair.

This patchset adds a new test to the cm_BkgDaemon() request
selection loop, cm_RequestWillBlock().  If a request will block it
is skipped.  If there are no requests to process that would not have
blocked, the worker will sleep for 25ms instead of the usual 1s.

For BkgStore operations, the CM_SCACHEFLAG_DATASTORING flag is
used to indicating a blocking state.

For BkgFetch and PreFetch operations, the CM_BUF_WRITING and
CM_BUF_READING flags on the first cm_buf_t of the range is used
to indicate a blocking state.

Change-Id: I95d9d1f14dbe0c7d717e6a7253ccfb10a9fac851
Reviewed-on: http://gerrit.openafs.org/6056
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>