1) 'stix_data' is now optional; you can set up a MemorySink without
needing a starting set of data.
2) Removed stix2-validator calls. The validator fails when called on our
_STIXBase-derived classes because we store timestamps as datetime
objects, while the validator expects strings. Also, this was the only
datastore that used the validator, and we should be consistent across
all of them. The validator can be added back in later.
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