Add some context to the handlers (e.g. log)

[karalabe]

Currently in v1 the service/topic handlers work solely based on the binary data arriving from the network (broadcast, request, event). The issue is, that in the current API it is impossible to transfer the request context from the service layer up to the handler (think in terms of the contextual logger).

In the Java binding, the logger context can be carrier to the handler through the ThreadLocal, so it is covered, however Go doesn't have the notion of goroutine state, so maybe the binary blobs should be wrapped in some contextual struct containing the logger too.

Dunno, will have to ponder on this a bit.