teach store.Store about CompareAndDelete
track CompareAndDelete stats
add compareAndDelete event action
add the CompareAndDelete command
support CreateAndDelete through the v2 server
keep the Delete tests together
implement recursive for CompareAndDelete in the store
update the v2 server to support recursive on CompareAndDelete events
merge compareAndDelete
refactor(compareAndDelete)
tests(get_handler) loosen the time assumption for the un
refactor(store.go) handle short condition first
test(delete_handler_test.go) fix inconsistent between test case and comments
Merge pull request #408 from xiangli-cmu/compareAndDelete

Compare and delete