Cleaned up hexdumps in debug output.

Cleaned up hexdumps in debug output.

 - the ngx_quic_hexdump0() macro is renamed to ngx_quic_hexdump();
   the original ngx_quic_hexdump() macro with variable argument is
   removed, extra information is logged normally, with ngx_log_debug()

 - all labels in hex dumps are prefixed with "quic"

 - the hexdump format is simplified, length is moved forward to avoid
   situations when the dump is truncated, and length is not shown

 - ngx_quic_flush_flight() function contents is debug-only, placed under
   NGX_DEBUG macro to avoid "unused variable" warnings from compiler

 - frame names in labels are capitalized, similar to other places