Use the mutex in exvar.Set since map access is not atomic.
Imagine your var has a value of zero. If you have a goroutine calling Set(5),
and another calling Increment(+1), then you only want one of these outcomes:
- Set completes first, and then Increment occurs => 6
- Increment completes first, and then Set occurs => 5
However, you could get a sequence:
- read (for Increment) 0
- set (for Set) 5
- write (for Increment) 1
This results in a value of 1, which is undesirable.