Remove a few unnecessary allocations.

Motivation:

Unnecessary allocations are bad!

Modifications:

- Use 'removeAll' instead of allocating a new buffer when dropping
  pending reads in the 'HTTP2StreamChannel'
- Use a static empty buffer in 'DataBuffer.evacuatePendingWrites'

Result:

- Fewer allocations.
- Resolves #240
- Resolves #241

Sources/NIOHTTP2/Frame Buffers/OutboundFlowControlBuffer.swift

@@ -250,6 +250,9 @@ private struct DataBuffer {
         return self.bufferedChunks.hasMark
     }
 
+    /// An empty buffer, we use this avoid an allocation in 'evacuatePendingWrites'.
+    private static let emptyBuffer = MarkedCircularBuffer<BufferElement>(initialCapacity: 0)
+
     init() {
         self.bufferedChunks = MarkedCircularBuffer(initialCapacity: 8)
         self.flushedBufferedBytes = 0
@@ -307,7 +310,7 @@ private struct DataBuffer {
 
     /// Removes all pending writes, invalidating this structure as it does so.
     mutating func evacuatePendingWrites() -> MarkedCircularBuffer<BufferElement> {
-        var buffer = MarkedCircularBuffer<BufferElement>(initialCapacity: 0)
+        var buffer = DataBuffer.emptyBuffer
         swap(&buffer, &self.bufferedChunks)
         return buffer
     }

Sources/NIOHTTP2/HTTP2StreamChannel.swift

@@ -620,8 +620,8 @@ final class HTTP2StreamChannel<Message: HTTP2FramePayloadConvertible & HTTP2Fram
 private extension HTTP2StreamChannel {
     /// Drop all pending reads.
     private func dropPendingReads() {
-        /// To drop all the reads, as we don't need to report it, we just allocate a new buffer of 0 size.
-        self.pendingReads = CircularBuffer(initialCapacity: 0)
+        /// We don't need to report the dropped reads, just remove them all.
+        self.pendingReads.removeAll()
     }
 
     /// Deliver all pending reads to the channel.