pions / webrtc / 2630

Pull #541

travis-ci

Optimize ice.Agent.getBestPair()

This function should be trivial but profiles show it taking 3-5% of my
application's CPU time. The problem is the constant creation of a
channel for message passing. This function is called by srtp.WriteRTP
for each packet, so it adds up.

My fix was to create a `runInner` function that utilizies a mutex. I
wanted to convert all `run` functions to `runInner` but the code
occasionally deadlocks in unit tests. The package is definitely a
candidate for a rewrite.

```
name           old time/op    new time/op    delta
GetBestPair-8     915ns ± 3%      80ns ± 4%  -91.23%  (p=0.008 n=5+5)

name           old alloc/op   new alloc/op   delta
GetBestPair-8      128B ± 0%       16B ± 0%  -87.50%  (p=0.008 n=5+5)

name           old allocs/op  new allocs/op  delta
GetBestPair-8      3.00 ± 0%      1.00 ± 0%  -66.67%  (p=0.008 n=5+5)
```

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