Improvements to -dataForColumnIndex:/-dataForColumn:

SQLite docs say sqlite3_column_blob should be called before
sqlite_3_column_bytes in case the data has to be converted.
Also allocate the NSData more efficiently.
Comment out FMDB code that’s unused by Couchbase Lite

There’s a lot of FMDB that Couchbase Lite doesn’t use, but Objective-C
methods can’t be dead-stripped by the linker. So I’ve gone and commented
out the stuff we don’t use, to lower our code size.
Added “CBL_” prefix to class names to avoid conflicts in apps

If an app uses Couchbase Lite and regular FMDB, this way the FMDB
classes won’t conflict and break everything.