Fix PATH validation, STOP_SENDING and memory leaks

• Improve path validation logic to avoid sending padded packet to unverified peer, follow 3X anti-amplification rule.
• Optimize application callback when STOP_SENDING frame is received.
• Improve new connection callback logic.
• Improve handling of a closed connection.
• Update BoringSSL to the most recent version.
• Fix memory leaks in gquic handshake and packet handling.
• Update ls-qpack to 2.5.1, ls-hpack to 2.3.1 to address compiler warnings