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
#435 Remove reduce test accoarding to code review

Per code review, removing reduce test as the callReduce method will return nil when there is no reduce. The code is also cleaner without the test.