Reject long sequences of CONTINUATION frames

[clintonpi]

Motivation:

Long sequences of CONTINUATION frames can be used to mount attacks by attempting to get a remote peer to consume large amounts of memory.

Modifications:

• Add a limit to the number of sequential CONTINUATION frames that can be received. This limit is configurable by users at the NIOHTTP2Handler level and has a default value of 5. When this limit is exceeded, the recipient responds with a GOAWAY frame and an "Enhance Your Calm" error of a newly created type ExcessiveContinuationFrames.

Result:

Long sequences of CONTINUATION frames are now rejected by the recipient.

[glbrntt]

@swift-server-bot add to allowlist