[Reviewer: Rob] cpp-comon infrstructure changes

Rob, please can you review these infrastructure changes needed for the memento cassandra store. Hightlights are:

* The biggest changes are to `cass_test_utils.h` and `mock_cassandra_store.h`. These consist almost entirely of code taken from homestead, though `SliceDeltionMatcher` and `EXPECT_DO_ASYNC` are new. 
* Some minor teaks to store classes, including addition of a `delete_slice` utility method. 
* Some enhancements to SAS logging infrastructure. 

Tested as part of homestead and memento UTs. 
Shouldn't have Homestead-specific comments in cpp-common - instead, we should be explicit that you can't call this for non-ASCII strings.