Add lsquic_conn_get_sni()

• [API] Add lsquic_conn_get_sni(), fixes issue #203.

Bug fixes: cancel challenges, delay critical stream destruction, et al

• [BUGFIX] gQUIC: do not destroy critical streams when connection is closed. See issue #201.
• [BUGFIX] Drop #if LSQUIC_CONN_STATS from lsquic.h. See issue #211.
• [BUGFIX] Challenge cancellation when path validation fails.
• [BUGFIX] Do not send FIN if RST is scheduled to be sent on a stream.
• [BUGFIX] gQUIC's is_tickable() when connection is closing.
• [BUGFIX] Q050 processing of GOAWAY frames.

Fix memory corruption in receive history copy-ranges

• [BUGFIX] Memory corruption in receive history copy-ranges function.

API to set batch size; a few small fixes

• [API] New knob to set outgoing packet batch size.
• Aborted connection now become tickable immediately.
• Abort connection when HTTP/3 frame cannot be opened (can only happen when malloc fails).

Fix memory corruption in receive history copy-ranges

• [BUGFIX] Memory corruption in receive history copy-ranges function.

Drop keylog APIs; improve ALPN debug logging

• [API] Remove keylog callbacks. See issue #188.
• Add a bit more ALPN logging.

Several stop-sending bug fixes; new optional callback

• [BUGFIX] Do not drop incoming data when STOP_SENDING is received.
• [BUGFIX] Receipt of STOP_SENDING should not cause read-reset.
• [BUGFIX] Allow stream writes after receiving RESET.
• [BUGFIX] Typo in stream: ANDing enum with wrong flag.
• [BUGFIX] Reset elision: do not use zero as special stream ID value, for zero is a valid stream ID in IETF QUIC.
• [API] Add optional on_conncloseframe_received() callback.
• Use zero error code in RESET stream sent in response to STOP_SENDING.

Fix migration corner cases and stream unit test

• [BUGFIX] Migration corner cases: drop or pad over path challenge and response frames when necessary.
• Fix stream unit test.

Handle packet reordering; add parameter to get_ssl_ctx() callback

• [OPTIMIZATION] Adjust packet reordering threshold when spurious losses are detected.
• [API] Pass pointer to local sockaddr to ea_get_ssl_ctx() callback.

Option to delay on_close; new on_reset callback

• [API, FEATURE] Add es_delay_onclose option to delay on_close until all data is ACKed. Use new function lsquic_stream_has_unacked_data() to learn whether peer acknowledged all data written to stream.
• [API] Add optional on_reset() stream callback to get notifications when RESET or STOP_SENDING frames are received.
• [BUGFIX] On STOP_SENDING, make conn tickable is writeable, not readable.