Check for stale caches in detchar cache creation

Sometimes, data is not distributed to /dev/shm/kafka, but the cache is still full of old data. We previously only checked if the start time of interest was after the cache start time, which is satisfied if old data is still sitting around in the cache. We now also check that the end time of interest is also contained in the cache, so that we try another data discovery method (i.e., gwdatafind) if the cache is stale.

The test_omegascan_skips_ew change is to account for the restructuring of the `create_cache` function. Previously, this test tried to create an omegascan, which involves creating a cache, etc. Since all we want to test with this is that we are waiting until data arrives for early warning events, we can patch out the rest of the test.