crimson: manage log submission atomicity independently of pipeline stages

crimson: manage log submission atomicity independently of pipeline stages

It's important to construct and submit log entries atomically because
the submission order needs to match the entry numbering.  Previously,
this occurred under the pg-wide exclusive process stage.  Shortly, each
obc will have its own process stage, so we need to ensure that atomicity
seperately from the pipeline stages.  Introduce a simple mutex.

Signed-off-by: Samuel Just <sjust@redhat.com>