SocketPlugin: cache proxy decisions; improvements
SocketPlugin: cache proxy decisions; improvements

Cache CORS proxy decisions as discussed in #63.
Also improve SocketPlugin code.

Closes #63

sebastianconcept/SqueakJS,sebastianconcept/SqueakJS,Speed-Pad/online,fniephaus/SqueakJS,the-impossible-quiz-revisited/the-impossible-quiz-revisited.github.io,bertfreudenberg/SqueakJS,bertfreudenberg/SqueakJS,bertfreudenberg/SqueakJS,fniephaus/SqueakJS,Speed-Pad/online,fniephaus/SqueakJS,the-impossible-quiz-revisited/the-impossible-quiz-revisited.github.io,sebastianconcept/SqueakJS,the-impossible-quiz-revisited/the-impossible-quiz-revisited.github.io,fniephaus/SqueakJS,the-impossible-quiz-revisited/the-impossible-quiz-revisited.github.io,bertfreudenberg/SqueakJS,Speed-Pad/online