CBQE-0: add more debug info to getting stats

Change-Id: I72dec5b3e1ba4b9a7f32f49e9472288347efbf8c
Reviewed-on: http://review.couchbase.org/32586
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>