Combine the two NIOAsyncChannel channel handlers (#2779)

Motivation:

The NIOAsyncChannel allocates 12 times on init. 4 of these allocations
come from creating two channel handlers and two channel handler
contexts. There's no inherent reason that these channel handlers can't
be combined to eliminate two allocations (one handler and one context).

Modifications:

- Combine `NIOAsyncChannelInboundStreamChannelHandler` and
  `NIOAsyncChannelOutboundWriterHandler` into a single
  `NIOAsyncChannelHandler`. Most of this was straightforward as only a
  few handler operations were duplicated across both.
- Add a 'NIOAsyncChannelHandlerWriterDelegate' in place of the
  'NIOAsyncChannelOutboundWriterHandler.Delegate'. One knock on from
  this is that the new delegate stores callbacks rather than the
  concrete type of the handler. This is necessary to prevent the
  generics from the new channel handler bubbling up to the outbound
  writer (which would break API and be somewhat odd).

Result:

Fewer allocations

Benchmarks/Thresholds/5.10/NIOCoreBenchmarks.NIOAsyncChannel.init.p90.json

@@ -1,3 +1,3 @@
 {
-  "mallocCountTotal" : 12
+  "mallocCountTotal" : 10
 }

Benchmarks/Thresholds/5.8/NIOCoreBenchmarks.NIOAsyncChannel.init.p90.json

@@ -1,3 +1,3 @@
 {
-  "mallocCountTotal" : 12
+  "mallocCountTotal" : 10
 }

Benchmarks/Thresholds/5.9/NIOCoreBenchmarks.NIOAsyncChannel.init.p90.json

@@ -1,3 +1,3 @@
 {
-  "mallocCountTotal" : 12
+  "mallocCountTotal" : 10
 }

Benchmarks/Thresholds/nightly-main/NIOCoreBenchmarks.NIOAsyncChannel.init.p90.json

@@ -1,3 +1,3 @@
 {
-  "mallocCountTotal" : 12
+  "mallocCountTotal" : 10
 }

Benchmarks/Thresholds/nightly-next/NIOCoreBenchmarks.NIOAsyncChannel.init.p90.json

@@ -1,3 +1,3 @@
 {
-  "mallocCountTotal" : 12
+  "mallocCountTotal" : 10
 }

Sources/NIOCore/AsyncChannel/AsyncChannel.swift

@@ -315,16 +315,27 @@ extension Channel {
     ) throws -> (NIOAsyncChannelInboundStream<Inbound>, NIOAsyncChannelOutboundWriter<Outbound>) {
         self.eventLoop.assertInEventLoop()
 
-        let inboundStream = try NIOAsyncChannelInboundStream<Inbound>.makeWrappingHandler(
-            channel: self,
+        let handler = NIOAsyncChannelHandler<Inbound, Inbound, Outbound>(
+            eventLoop: self.eventLoop,
+            transformation: .syncWrapping { $0 },
+            isOutboundHalfClosureEnabled: isOutboundHalfClosureEnabled
+        )
+
+        let inboundStream = try NIOAsyncChannelInboundStream(
+            eventLoop: self.eventLoop,
+            handler: handler,
             backPressureStrategy: backPressureStrategy,
             closeOnDeinit: closeOnDeinit
         )
+
         let writer = try NIOAsyncChannelOutboundWriter<Outbound>(
-            channel: self,
+            eventLoop: self.eventLoop,
+            handler: handler,
             isOutboundHalfClosureEnabled: isOutboundHalfClosureEnabled,
             closeOnDeinit: closeOnDeinit
         )
+
+        try self.pipeline.syncOperations.addHandler(handler)
         return (inboundStream, writer)
     }
 
@@ -338,17 +349,27 @@ extension Channel {
     ) throws -> (NIOAsyncChannelInboundStream<ChannelReadResult>, NIOAsyncChannelOutboundWriter<Never>) {
         self.eventLoop.assertInEventLoop()
 
-        let inboundStream = try NIOAsyncChannelInboundStream<ChannelReadResult>.makeTransformationHandler(
-            channel: self,
+        let handler = NIOAsyncChannelHandler<Channel, ChannelReadResult, Never>(
+            eventLoop: self.eventLoop,
+            transformation: .transformation(channelReadTransformation: channelReadTransformation),
+            isOutboundHalfClosureEnabled: isOutboundHalfClosureEnabled
+        )
+
+        let inboundStream = try NIOAsyncChannelInboundStream(
+            eventLoop: self.eventLoop,
+            handler: handler,
             backPressureStrategy: backPressureStrategy,
-            closeOnDeinit: closeOnDeinit,
-            channelReadTransformation: channelReadTransformation
+            closeOnDeinit: closeOnDeinit
         )
+
         let writer = try NIOAsyncChannelOutboundWriter<Never>(
-            channel: self,
+            eventLoop: self.eventLoop,
+            handler: handler,
             isOutboundHalfClosureEnabled: isOutboundHalfClosureEnabled,
             closeOnDeinit: closeOnDeinit
         )
+
+        try self.pipeline.syncOperations.addHandler(handler)
         return (inboundStream, writer)
     }
 }