@_callbackDeferreds is collecting garbage from no-return-value calls

[radex]

All calls to Goldengate.dispatch create a Deferred object and save it onto a map. Problem is, if the call doesn't return anything, the Deferred stays there forever.

Unlikely to be a real problem in most applications, but a memory leak nonetheless.

If we do #1, the fix we'll be trivial — we'll know on JS side which methods never return a value and just never create or return the deferred.