New API calls and bug fixes

• [API] lsquic_ssl_sess_to_resume_info() is the new way to get session info.
• [API] Add user pointer to ea_generate_scid callback.
• [API] Add lsquic_dcid_from_packet() -- a fast function to parse out DCID.
• [API] Add es_max_batch_size to control outgoing packet batch size.
• [BUGFIX] Disallow sending of header while promise is being written.
• [BUGFIX] Flush stream when buffered bytes exhaust stream cap.
• [BUGFIX] Deactivate HQ frame if writing push promise fails.
• Perform sanity check on peer transport parameters and fail the handshake if some flow control limits are too low. This can be turned off, see es_check_tp_sanity.
• http_server: fix how requests are read in "hq" mode.