EX-4408 lipe: collect some statistic data with lpcc_purge

EX-4408 lipe: collect some statistic data with lpcc_purge

Collect the number of cached files, min/max/avg file size,
min/max/avg age of files in LPCC. Scan cache device forcefully
even if it is not full enough to collect the statistics data
in time.

Change-Id: Id716d4689c83ecc5754e41734e44e7c051d36a8e
Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial testlist=sanity-pcc
Reviewed-on: https://review.whamcloud.com/45937
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>