Avoid CoW while processing the next state when HTTP2FrameDecoder decodes

[stefanadranca]

Motivation:

CoWs appear when switching over the current state of the parser and holding it while also modifying it - inside processNExtState(). Following append(bytes: ByteBuffer)'s pattern, we should use a function that sets the state to an intermediary one with no associated data, before making the transformations.

Modifications:

• updated the avoidParserCoW() helper function to take into consideration throwing functions
• used it in the switch cases over the parser state inside processNextState()

Result:

CoW will be avoided when changing the state of the HTTP2FrameDecoder while decoding, so we won't have unnecessary heap allocations.

[glbrntt]

@swift-server-bot add to allowlist