[#8] limiting: Make SemaphoreLimiter.Acquire() zero-alloc

Previously, `Acquire` on exising key did 1 allocation because
`func() { sem.Release() }` was a closure capturing different variables.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

limiting/limiter.go

@@ -69,7 +69,7 @@ func (lr *SemaphoreLimiter) Acquire(key string) (ReleaseFunc, bool) {
 	}
 
 	if ok := sem.Acquire(); ok {
-		return func() { sem.Release() }, true
+		return sem.Release, true
 	}
 	return nil, false
 }