MINOR: quic: reduce half open counters scope

MINOR: quic: reduce half open counters scope

Accounting is implemented for half open connections which represent QUIC
connections waiting for handshake completion. When reaching a certain
limit, Retry mechanism is automatically activated prior to instantiate
new connections.

The issue with this behavior is that two notions are mixed : QUIC
connection handshake phase and Retry which is mechanism against
amplification attacks. As such, only peer address validation should be
taken into account to activate Retry protection.

This patch chooses to reduce the scope of half_open_conn. Now only
connection waiting to validate the peer address are now accounted for.
Most notably, connections instantiated with a validated Retry token
check are not accounted.

One impact of this patch is that it should prevent to activate Retry
mechanism too early, in particular in case if multiple handshakes are
too slow. Another limitation should be implemented to protect against
this scenario.