Avoid creating a yield ID counter per async writer

Motivation:

The NIOAsyncWriter uses an atomic to generate yield IDs. Each writer has
its own atomic. We can save an allocatiuon per writer but using a shared
atomic.

Each load then wrapping increment operation with relaxed ordering takes
approx 3ns on my machine. For a UInt64 this would take approx 188 years
to wrap around if run in a tight loop.

Modification:

- Share a single yield ID counter for NIOAsyncWriter

Result:

Fewer allocations

Author: glbrntt

Sources/NIOCore/AsyncSequences/NIOAsyncWriter.swift

@@ -17,6 +17,9 @@ import DequeModule
 import NIOConcurrencyHelpers
 import _NIODataStructures
 
+@usableFromInline
+let yieldIDCounter = ManagedAtomic<UInt64>(0)
+
 /// The delegate of the ``NIOAsyncWriter``. It is the consumer of the yielded writes to the ``NIOAsyncWriter``.
 /// Furthermore, the delegate gets informed when the ``NIOAsyncWriter`` terminated.
 ///
@@ -435,13 +438,12 @@ extension NIOAsyncWriter {
             }
 
             @usableFromInline
-            /* private */ internal let _yieldIDCounter = ManagedAtomic<UInt64>(0)
 
             @inlinable
             func generateUniqueYieldID() -> YieldID {
                 // Using relaxed is fine here since we do not need any strict ordering just a
                 // unique ID for every yield.
-                .init(value: self._yieldIDCounter.loadThenWrappingIncrement(ordering: .relaxed))
+                .init(value: yieldIDCounter.loadThenWrappingIncrement(ordering: .relaxed))
             }
         }