Fixes overflow panic in stream counters

[nullsauce]

Hey. This should fix #7

I've implemented the wrapped adds for the two counters inside a dedicated struct so that the behavior is isolated and easy to test. Apart from running the test suite, is there anything else I should check ? I'm new to the project, any comments / directions are welcome.

I've put the tests in sender_test.rs for consistency but would have preferred putting them in sender_stream.rs to avoid having to make Counter pub(crate) visible and make Counter::mock a test helper function. What do you think ?

Edit: Tested this fix with webrtc-rs/webrtc 0.4.0 and I was able to display a 4k 30fps h264 stream in Firefox for more than 3 hours without any overflow panic happening.

[xnorpx]

@k0nserv would be nice to get this in.

[k0nserv]

On holiday atm, will be back on the 16th of August. Please ask someone else in with merge permissions if it's urgent

[xnorpx]

@rainliu thanks for merge can we do patch release for this?