LU-56 ptlrpc: svc thread starting/stopping cleanup

LU-56 ptlrpc: svc thread starting/stopping cleanup

This patch covered two things:
- serialize creation of ptlrpc service thread
  In current version we can parallel create service threads, so there
  could be "hole" of thread ID if one creation failed, it could be
  problemaic because some modules require thread ID to be strictly
  contiguous and unique. Serializing thread creation can resolve this
  issue.
- code cleanup for for stopping of ptlrpc servcie threads, this is
  just for the next step work of partitioned ptlrpc service.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: Ied8ad89003aa9d53fa73a4e5166a2c8d07a1aae9
Reviewed-on: http://review.whamcloud.com/2912
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: wangdi <di.wang@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>