Windows: Insert Server Reference List changes

Windows: Insert Server Reference List changes

When inserting a new cm_serverRef_t object into a server list
perform the following operations:

1. take advantage of the fact that the cm_serverLock is held
   exclusively to purge the list of any deleted entries that
   could not be removed previously.

2. check to ensure that the item that is being added does not
   already exist in the list.  If it does, discard it.

Change-Id: Ibabfc3b2e1b716f2a0cc664a4667bc9219fde09c
Reviewed-on: http://gerrit.openafs.org/5258
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementix.org>