NetworkPreference.best should be POSIX on older platforms (#896)
NetworkPreference.best should be POSIX on older platforms (#896)

Motivation:

While NetworkPreference.best was supposed to abstract away the choice of
exactly which event loop would be used, on older Darwin platforms it
would actually lead to runtime crashes as it would try to use an
implementation it didn't have.

Modifications:

Have .best's .implementation fall back to POSIX on older platforms.

Result:

Fewer crashes on older platforms.
grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift