Add improved timeout handling for HTTP transactions.

Introduces AIHTTPTimeoutPolicy objects which do not just
specify a single "timeout" in seconds, but a plethora of
timings related to the life cycle of the average HTTP
transaction.

This knowledge is that moved to the Responder being
used instead of floating constants hardcoded in the
callers of http requests. This assumes that the same
timeout policy is wanted for each transaction that
uses the same Responder, which can be enforced is needed.

I added a AIHTTPTimeoutPolicy for EVERY responder,
only to make it easier later to tune timeout values
and/or to get feedback about which responder runs
into HTTP errors in debug output (especially time outs),
so that they can be tuned later. If we already understood
exactly what we were doing then most responders could
have been left alone and just return the default timeout
policy: by far most timeout policies are just a copy
of the default policy, currently.

This commit is not finished... It's a work in progress
(viewer runs fine with it though).

indra/llmessage/llsdmessage.cpp

@@ -77,17 +77,18 @@ bool LLSDMessage::httpListener(const LLSD& request)
         out << "request event without 'url' key to '" << mEventPump.getName() << "'";
         throw ArgError(out.str());
     }
+#if 0	// AIFIXME: ignore this for now
     // Establish default timeout. This test relies on LLSD::asReal() returning
     // exactly 0.0 for an undef value.
     if (! timeout)
     {
         timeout = HTTP_REQUEST_EXPIRY_SECS;
     }
+#endif
     LLHTTPClient::post4(url, payload,
                        new LLSDMessage::EventResponder(LLEventPumps::instance(),
                                                        request,
-                                                       url, "POST", reply, error),
-                       (F32)timeout);
+                                                       url, "POST", reply, error));
     return false;
 }