- [API] Rename "0-RTT" to "session resumption." In IETF QUIC, "0-RTT"
  always refers to early data, meaning a request that the server can
  reply to in the very first return flight.  A more appropriate name
  for what we support on the client site is "session resumption," which
  is standard TLS terminology.  Later, when we add support for 0-RTT
  (early data), we can use the 0-RTT terminology again, this time in
  proper context.
- [BUGFIX] Do not set certificate callback if ea_lookup_cert is NULL.
- [BUGFIX] Make connection tickable when it's marked as closed.
- [BUGFIX] Fail certificate lookup if SNI is not present in HTTP mode.
- Several documentation fixes and improvements.
- Minor code cleanup.