Golang: do something with context.Context

PR Here: https://github.com/apache/thrift/pull/1459

This patch wires through context.Context such that it can be used in in http.Request's WithContext method. This allows Thrift HTTP requests to canceled or timed out via the context.

This patch breaks support for go<1.7 so it's not ready to ship, but I'm hoping to get some direction on this. When does Thrift expect to drop support of go1.7? It looks like the current solution is to duplicate files that need to use golang.org/x/net/context and add a // +build !go1.7 but duplication seems unsustainable as the context package is imported more places.

Go 1.7 was released 15 August 2016. Given Golang has had significant performance improvements in most dot releases, I suspect most production services stay reasonably up to date. Here at Periscope/Twitter we're on go1.9.1, and we're a fairly large organization.