MGM: Http - Allow stalling for HTTP if EOS_MGM_ALLOW_HTTP_STALL is set

The CTA team wanted a way to prevent users from writing via HTTP. The entire stalling mechanism was disabled in HTTP by the following commit d0aab68a as it had behaviors that the HTTP protocol cannot deal with currently (for example, the server cannot currently ask the client to come back later because it is busy).

The stalling mechanism for HTTP can only be enabled via the EOS_MGM_ALLOW_HTTP_STALL mechanism.

Fixes EOS-6200

Closes EOS-6200