Avoid 2 heap allocations on child channel creation (apple#115)

Motivation:

When users create a child channel and want to get hold of it,
they will pass a promise to createStreamChannel. This promise
will force a call to map{} and a call to cascade(), both of
which heap allocate. We don't need to have these heap allocations,
and they're easily removed, so we should.

Modifications:

Manually cascade the result.

Result:

Slightly fewer heap allocations in some workloads.

Sources/NIOHTTP2/HTTP2StreamChannel.swift

@@ -100,8 +100,7 @@ final class HTTP2StreamChannel: Channel, ChannelCore {
         self._pipeline = ChannelPipeline(channel: self)
     }
 
-    @discardableResult
-    internal func configure(initializer: ((Channel, HTTP2StreamID) -> EventLoopFuture<Void>)?) -> EventLoopFuture<Void> {
+    internal func configure(initializer: ((Channel, HTTP2StreamID) -> EventLoopFuture<Void>)?, userPromise promise: EventLoopPromise<Channel>?){
         // We need to configure this channel. This involves doing four things:
         // 1. Setting our autoRead state from the parent
         // 2. Calling the initializer, if provided.
@@ -116,6 +115,9 @@ final class HTTP2StreamChannel: Channel, ChannelCore {
             if self.parent!.isActive {
                 self.performActivation()
             }
+
+            // We aren't using cascade here to avoid the allocations it causes.
+            promise?.succeed(self)
         }
 
         f.whenFailure { (error: Error) in
@@ -124,8 +126,9 @@ final class HTTP2StreamChannel: Channel, ChannelCore {
             } else {
                 self.errorEncountered(error: error)
             }
+
+            promise?.fail(error)
         }
-        return f
     }
 
     /// Activates this channel.

Sources/NIOHTTP2/HTTP2StreamMultiplexer.swift

@@ -64,7 +64,7 @@ public final class HTTP2StreamMultiplexer: ChannelInboundHandler, ChannelOutboun
                                              targetWindowSize: 65535,
                                              initiatedRemotely: true)
             self.streams[streamID] = channel
-            channel.configure(initializer: self.inboundStreamStateInitializer)
+            channel.configure(initializer: self.inboundStreamStateInitializer, userPromise: nil)
             channel.closeFuture.whenComplete { _ in
                 self.childChannelClosed(streamID: streamID)
             }
@@ -184,14 +184,10 @@ extension HTTP2StreamMultiplexer {
                                              targetWindowSize: 65535,  // TODO: make configurable
                                              initiatedRemotely: false)
             self.streams[streamID] = channel
-            let activationFuture = channel.configure(initializer: streamStateInitializer)
+            channel.configure(initializer: streamStateInitializer, userPromise: promise)
             channel.closeFuture.whenComplete { _ in
                 self.childChannelClosed(streamID: streamID)
             }
-
-            if let promise = promise {
-                activationFuture.map { channel }.cascade(to: promise)
-            }
         }
     }
 }