Remove DispatchGroup and replace with condvar (#2687)

Motivation:

DispatchGroup can cause thread performance checker warnings in
places we don't want them. We can avoid those warnings in some
cases by using a condition variable instead.

Modifications:

- Replaced a use of DispatchGroup with a condition variable
    in NIOThreadPool setup.
- Removed an unnecessary DispatchGroup in the shutdown code

Result:

Fewer thread performance checker warnings.

Sources/NIOPosix/NIOThreadPool.swift

@@ -93,7 +93,7 @@ public final class NIOThreadPool {
  }
  return
  }
- let g = DispatchGroup()
+
  let threadsToJoin = self.lock.withLock { () -> [NIOThread] in
  switch self.state {
  case .running(let items):
@@ -117,12 +117,11 @@ public final class NIOThreadPool {
  }
  }
 
- DispatchQueue(label: "io.swiftnio.NIOThreadPool.shutdownGracefully").async(group: g) {
+ DispatchQueue(label: "io.swiftnio.NIOThreadPool.shutdownGracefully").async {
  threadsToJoin.forEach { $0.join() }
- }
-
- g.notify(queue: queue) {
- callback(nil)
+ queue.async {
+ callback(nil)
+ }
  }
  }
 
@@ -233,7 +232,10 @@ public final class NIOThreadPool {
  return
  }
 
- let group = DispatchGroup()
+ // We use this condition lock as a tricky kind of semaphore.
+ // This is done to sidestep the thread performance checker warning
+ // that would otherwise be emitted.
+ let cond = ConditionLock(value: 0)
 
  self.lock.withLock {
  assert(self.threads == nil)
@@ -242,19 +244,21 @@ public final class NIOThreadPool {
  }
 
  for id in 0..<self.numberOfThreads {
- group.enter()
  // We should keep thread names under 16 characters because Linux doesn't allow more.
  NIOThread.spawnAndRun(name: "\(threadNamePrefix)\(id)", detachThread: false) { thread in
  self.lock.withLock {
  self.threads!.append(thread)
+ cond.lock()
+ cond.unlock(withValue: self.threads!.count)
  }
- group.leave()
+
  self.process(identifier: id)
  return ()
  }
  }
 
- group.wait()
+ cond.lock(whenValue: self.numberOfThreads)
+ cond.unlock()
  assert(self.lock.withLock { self.threads?.count ?? -1 } == self.numberOfThreads)
  }