EX-3715 pcc: add stats for attach|detach|auto_attach

EX-3715 pcc: add stats for attach|detach|auto_attach

In this patch, we add stats for PCC attach, detach and
auto_attach.
With this feature, we verify that PCC can auto-attach the file
into PCC cache without having to re-fetch the data of the whole
file.
Add sanity-pcc test_44.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Ia0c1cd6b414998e72859aaf34c125b5a4e4e743c
Reviewed-on: https://review.whamcloud.com/44764
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>