[PATCH 0 of 2] QUIC connection reuse

[Roman Arutyunyan]

- patch #1 adds support for reusable and idle modes for the main QUIC
  conneciton.  They allow to close QUIC connections that have no active
  request streams when connections are needed for a new client or when nginx
  worker is shutting down.

- patch #2 allows to switch main QUIC connection to reusable and idle modes
  after handshake and util streams are created.  A new callback init_streams()
  is added for this.