Remove `sync::Context` trait. (#3105)

Instead of passing a context around to each method, thereby introducing
side-effecting I/O actions everywhere, with this PR `sync::ChainSync`
only contains state which is updated by invoking various callback
methods (`on_*`) and actionable items are returned as regular results
from method calls, often iterators yielding requests that should be
issued to peers. It is up to the caller to handle these in an
appropriate way, currently `protocol` will send those as messages.