Fixed more unexported-symbol issues

Missed CBLHTTPErrorDomain (an NSString constant).
Also, I renamed the classes GCDAsyncSocket and WebSocket to have a "CBL_"
prefix but forgot to update them in the .exp file.
For some reason both of these mistakes only manifested as errors in the
Release build, not Debug -- I thought the symbol exporting would be the
same in both cases but apparently not.
Fixed a Release-only compile error in CBLQueryPlanner