GWMessageHandlerImpl: fix Poco before 1.9
GWMessageHandlerImpl: fix Poco before 1.9

Method Poco::JSON::Object::getNames() is available since Poco 1.9.
Building with older versions would fail. Workaround by iterating
and creating a vector of names manually.

Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server