Add Throttle and LogThrottle classes to manage throttled APIs.

[nat-goodspeed]

No description provided.

[nat-goodspeed]

Seeing the pattern of functions being throttled, I've been thinking about such a wrapper for a while now. The usage is a little more awkward than I'd hoped. Is it worth pursuing this direction?

I'd like even better to have LogThrottle (or some other subclass) wrap a whole LLEventAPI subclass method, so we could directly pass the LogThrottle data member to LLEventAPI::add(). I haven't yet tried to make that work.

Another Throttle subclass I'd like to see would be one that knows about the void(const LLSD&) signature and uses sendReply() to notify the LEAP invoker that the call has been throttled.

[maxim-productengine]

I like this wrapper. I'm sure it will be handy with future apis too.