Fix http transport client blocking recv

[mapogolions]

I observed that the Recv call gets blocked in the default implementation of the HTTP transport client under certain conditions.

1. The issue is reproduced in the unit tests introduced in commit 2529793
2. The commit 9ee16e2 demonstrates a fix for the issue
3. The commit 182b736 fixes race condition
   The race condition lies in the fact that the http.Response may be associated with the wrong http.Request
   I created a separate branch in my repository to reproduce the race condition. I added a unit test. Here is the link. So, we need to adjust the scope of the lock to resolve the problem.