Limit ASIO session timeout to request timeout

Summary: We've seen requests get stuck on timedwait's in asio.  It's not
totally clear why they're getting stuck, but regardless it seems that we
shouldn't wait longer than the request timeout for something to return.

Reviewed By: @jano

Differential Revision: D1665567

Signature: t1:1665567:1415390539:e39ea95cfd07bc41781b198c7ef87c36db84fbec