quic: don't crash when no quic engine is available

quic: don't crash when no quic engine is available

- When there is no quic engine available, vpp crashes with an
  assert on startup which is not production friendly.
  Instead, emit warning message to stderr.
- Fix crash in session_transport_closing_notify if
  a remote closed notification happens on an invalid
  session in the quicly engine.
- Fix compilation errors when QUIC_DEBUG is set to 2
- Fix crash in 'show quic crypto context' command when
  quic engine is not present.
- Add quic engine type to 'show quic' output.

Type: fix

Change-Id: I83b9e69873fa3cd61d1d852b78ed1ccd2112825c
Signed-off-by: Dave Wallace <[email protected]>