Context/resource ID should be always provided in the hostcall #59

Currently, the effective context_id is tracked by the host, but this results in a weird asymmetry between hostcalls (without context_id) and callbacks (with context_id), and makes it harder to build higher-level interfaces on top of those hostcalls.

In order to help with migration from v0.2 to vNEXT, we should accept context_id=0 as "context_id tracked by the host". This should be advertised via settings/feature flag and eventually removed.