Before we were reading a boolean variable that was set by another goroutine.
Without any sync primitives, I think Go is within its rights to optimize
that away.
We now require that `ctx` is non-`nil`. This was already true for all callers
and consistent with other Go functions that take contexts.
This change incorporates feedback on https://github.com/pulumi/pulumi/pull/764,
in addition to refactoring the retry logic to use our retry framework rather
than hand-rolling it in the REST API code. It's a minor improvement, but at
least lets us consolidate some of this logic which we'll undoubtedly use more
of over time.
This change introduces a simple retry package which will be used in
resource providers in order to perform waits, backoffs, etc. It's
pretty basic right now but leverages the fancy new Golang context
support for deadlines and timeouts. Soon we will layer AWS-specific
acceptors, etc. atop this more general purpose thing.
