Added CBLQuery.mapOnly property

This was in CouchQuery but hadn't been ported yet, for some reason.
Allow setting CBLNewRevision property value to nil.
Changed weak-memory mode in CBLCache to avoid crashes

For some reason the "ZeroingWeakMemory" option doesn't work; it might
only support GC, not ARC. Switched to "WeakMemory", which does work.
Fixed regression where databases fail to open

"PUT /dbname" was failing with a 500. Underlying causes was a mixup due
to "open" being both a verb and an adjective :-p. Renamed the property
"isOpen" to disambiguate.