EX-4697 lipe: Define statistics fields for lpurge / lamigo

EX-4697 lipe: Define statistics fields for lpurge / lamigo

Added JSON stats output in lamigo
Extended JSON stats output in lpurge

Signed-off-by: Alexandre Ioffe <aioffe@ddn.com>
Test-Parameters: trivial testlist=hot-pools
Change-Id: Ib367022dd073c1699d75e3ea7cfa3b586e7b8877
Reviewed-on: https://review.whamcloud.com/48125
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Colin Faber <cfaber@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>