EX-5014 pcc: Change PCC commands to use constants

EX-5014 pcc: Change PCC commands to use constants

PCC command names are just written out as strings, making
them hard to track.  Change them all to use named commands.

This also includes a few minor debug and structural changes
as part of prep for the main patch against EX-5014.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Icad8dfdb44ed2562a95b2aaa0432cba221e4a1bc
Reviewed-on: https://review.whamcloud.com/46894
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>