Port RevIdCollator from iOS

- Original Code :
https://github.com/couchbase/couchbase-lite-ios/blob/master/Source/CBL_R
evision.m
- Renamed CouchbaseSqliteUnicodeCollationFunction to
CouchbaseSqliteJsonUnicodeCollationFunction
- Renamed CouchbaseSqliteAsciiCollationFunction to
CouchbaseSqliteJsonAsciiCollationFunction
- Renamed CouchbaseSqliteRawCollationFunction to
CouchbaseSqliteJsonRawCollationFunction
Merge pull request #15 from xamarin/port_revid_collator

Port RevIdCollator from iOS