Removed memory allocations from encryption code.

Removed memory allocations from encryption code.

 + ngx_quic_encrypt():
     - no longer accepts pool as argument
     - pkt is 1st arg
     - payload is passed as pkt->payload
     - performs encryption to the specified static buffer

 + ngx_quic_create_long/short_packet() functions:
    - single buffer for everything, allocated by caller
    - buffer layout is: [ ad | payload | TAG ]
      the result is in the beginning of buffer with proper length
    - nonce is calculated on stack
    - log is passed explicitly, pkt is 1st arg
    - no more allocations inside

 + ngx_quic_create_long_header():
    - args changed: no need to pass str_t

 + added ngx_quic_create_short_header()