First version of HTTP bandwidth throttling.

Adds throttling based on on average bandwidth usage per HTTP service.
Since only HTTP textures are using this, they are still starved by other
services like inventory and mesh dowloads. Also, it will be needed to
move the maximum number of connections per service the to the PerService
class, and dynamically tune them: reducing the number of connections is
the first thing to do when using too much bandwidth.

I also added a graph for HTTP texture bandwidth to the stats floater.
For some reason the average bandwidth (over 1 second) look almost like
scattered noise... weird for something that is averaged...

indra/llmessage/aihttptimeout.h

@@ -89,6 +89,7 @@ class HTTPTimeout : public LLRefCount {
 	U64 mStalled;								// The time (sTime_10ms) at which this transaction is considered to be stalling if nothing is transfered anymore.
   public:
 	static F64 const sClockWidth_10ms;			// Time between two clock ticks in 10 ms units.
+	static F64 const sClockWidth_40ms;			// Time between two clock ticks in 40 ms units.
 	static U64 sTime_10ms;						// Time since the epoch in 10 ms units.
 #if defined(CWDEBUG) || defined(DEBUG_CURLIO)
 	ThreadSafeBufferedCurlEasyRequest* mLockObj;