This branch updates the reconnect middleware to std::future.
Updating reconnect was pretty straightforward and mechanical, so it
should be easy to review.

Most of the complexity was actually in getting the test
retry_reconnect_errors which exercises this functionality to pass: the
test uses transport metrics to determine when the proxy has actually
accepted a connection from the test client, so that it can advance to
the next state; it was necessary to re-enable transport metrics for this
to work (see PR #530).

Additionally, there was a subtle change in the test-support client's
behavior that needed to be resolved as well: the test relied on
Client::request_async eagerly causing the test client to connect to
the proxy when called, rather than when the returned future is polled.
Switching request_async to an async fn broke this property — the
entire body of the function, even the synchronous parts, was run only
when the async fn future is polled. This means that the connection was
not opened when the test expected it to be, and the subsequent assertion
that it was open always failed.

Therefore, I changed request_async back into a synchronous function
returning impl Future. This is identical at the callsite (it still
returns something that you can .await), but calling the function now
once again eagerly tells the test client background thread to open a
connection. This fixes the test.

Depends on #530.