QUIC: switch stream context to a server selected by SNI.

QUIC: switch stream context to a server selected by SNI.

Previously the default server configuration context was used until the
:authority or host header was parsed.  This led to using the configuration
parameters like client_header_buffer_size or request_pool_size from the default
server rather than from the server selected by SNI.

Also, the switch to the right server log is implemented.  This issue manifested
itself as QUIC stream being logged to the default server log until :authority
or host is parsed.