QUIC: packet processing refactoring.

QUIC: packet processing refactoring.

All packet header parsing is now performed by ngx_quic_parse_packet()
function, located in the ngx_quic_transport.c file.

The packet processing is centralized in the ngx_quic_process_packet()
function which decides if the packet should be accepted, ignored or
connection should be closed, depending on the connection state.

As a result of refactoring, behavior has changed in some places:

 - minimal size of Initial packet is now always tested
 - connection IDs are always tested in existing connections
 - old keys are discarded on encryption level switch