[tgrabiec:
- extracted from a larger commit
- removed coupling with how cache_streamed_mutation is created (the
code went out of sync), used more stable make_reader(). it's simpler too.
- replaced false/true literals with is_continuous/is_dummy where appropraite
- dropped tests for cache::underlying (class is gone)
- reused streamed_mutation_assertions, it has better error messages
- fixed the tests to not create tombstones with missing timestamps
- relaxed range tombstone assertions to only check information relevant for the query range
- print cache on failure for improved debuggability
]
