Discard read bytes when accumulating continuation frames (#444)

Motivation:

When accumulating sequences of CONTINUATION frames, each frame is parsed
from a buffer. These bytes are read when the CONTINUATION frame is
parsed, but if more CONTINUATION frames follow then the buffer isn't
reset. This means that long sequences of CONTINUATION frames can result
in a larger than necessary buffer where most of the contents have
already been read.

Modifications:

- Discard the bytes of the accumulation buffer when transitioning back
  to AccumulatingHeaderBlockFragmentsParserState if more than half of
  the buffer has been read.

Result:

Lower memory footprint when parsing sequences of CONTINUATION frames.

Sources/NIOHTTP2/HTTP2FrameParser.swift

@@ -537,6 +537,19 @@ struct HTTP2FrameDecoder {
 
             // we have collected enough bytes: is this the last CONTINUATION frame?
             guard self.continuationHeader.flags.contains(.endHeaders) else {
+                // When accumulating a sequence of CONTINUATION frames the buffer will continue to
+                // grow as bytes are appended to it and the reader index will continue to advance
+                // as frames are read. However this allows for an ever increasing buffer where only
+                // the unread bytes are useful. The read bytes should be periodically discarded.
+                //
+                // As a basic heuristic is to discard the read bytes when more than 1KB has been
+                // read, and that 1KB accounts for more than half of the capacity.
+                let readerIndex = self.continuationPayload.readerIndex
+                let capacity = self.continuationPayload.capacity
+                if readerIndex > 1024 && readerIndex > (capacity / 2) {
+                    self.continuationPayload.discardReadBytes()
+                }
+
                 // nope, switch back to accumulating fragments
                 return .accumulateContinuationHeader(
                     AccumulatingHeaderBlockFragmentsParserState(