SERVER-15870 Prune command stats for commands that haven't run

Signed-off-by: Dan Pasette <dan@mongodb.com>
repair
SERVER-13635: allow kv engines to get index size and repair
Merge remote-tracking branch 'nextdoor/master'

Conflicts:
	src/mongo/db/storage/devnull/devnull_kv_engine.h
	src/mongo/db/storage/heap1/heap1_engine.h
	src/mongo/db/storage/kv/kv_engine.h
SERVER-13635: allow kv engines to get index size and repair
Merge remote-tracking branch 'nextdoor/master'
fix index size
fix index size