Use context consistently when it is part of a struct

[ChuntaoLu]

Context is sometimes attached to request struct (e.g. https://github.com/uber/zanzibar/blob/master/runtime/tchannel_server.go#L202) and is supposed to be used as the context for the request, but context is still passed in as a paramter to the methods whose receiver is the request (i.e., the request already have access to the context). This makes it possible that a request method uses a different context from the one it holds, e.g. https://github.com/uber/zanzibar/blob/master/runtime/tchannel_server.go#L246-L249. For structs that hold context internally, context should not be an argument to its methods.