MEDIUM: quic: limit handshake per listener

MEDIUM: quic: limit handshake per listener

Implement a limit per listener for concurrent number of QUIC
connections. When reached, INITIAL packets for new connections are
automatically dropped until the number of handshakes is reduced.

The limit value is automatically based on listener backlog, which itself
defaults to maxconn.

This feature is important to ensure CPU and memory resources are not
consume if too many handshakes attempt are started in parallel.

Special care is taken if a connection is released before handshake
completion. In this case, counter must be decremented. This forces to
ensure that member <qc.state> is set early in qc_new_conn() before any
quic_conn_release() invocation.