Inconsistent revision hash length

See CBL-242. Change rev id digest to use crc32c. There are already helper functions in place to generate this for body hash - would just need to be modified to incorporate all elements used when generating rev id digest (parent rev id, body, deleted flag). Review https://github.com/couchbase/couchbase-lite-core/pull/866/files to ensure the components used are the same as CBL. Can use CBL unit tests (C/tests/c4DocumentTest.cc) to validate rev id generation matches on the SG side.