MINOR: quic: move buffered ACK to streambuf

MINOR: quic: move buffered ACK to streambuf

QUIC streamdesc layer is used to manage QUIC MUX stream txbuf data
storage until acknowledgment. Currently, it only supports in-order
acknowledgment at the stream level. This requires to be able to buffer
out-of-order ACKs until they can be handled.

Previously, these ACKs were stored in a tree to the streamdesc instance.
Move this indexed storage at the streambuf instance.

This commit is purely an architecture change. However, it will allow to
extend ACK management in future patches, such as the ability to merge
overlapping out-of-order ACKs.