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.

Kudos to dnadasi for catching this.

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