Read request from context

[hannahhoward]

Goals

fix #367

• allows us to avoid a breaking API change
• Gives the ability for higher level layers to know the request ID AHEAD of calling NewRequest instead of registering an OutgoingRequestHook

Implementation

• Define a context key for other layers to set when calling Graphsync that allows specifying the request ID in the context
• if set, use the request ID from context rather than generate a new one.