1. Move logics related to creating endpoint, creating new connection and retry 0rtt connections into a wrapper construct QuicNewConnection to put the logic together.
2. get_or_add_connection: logic is much simplified to allow manage the connection cache -- the QUIC connection is lazy constructed. get_or_add_connection should no longer be a global hotspot.
3. Per connection stats update is moved out of get_or_add_connection to avoid bad connection impacting good connections.
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