The module implements random load-balancing algorithm with optional second
choice.  In the latter case, the best of two servers is chosen, accounting
number of connections and server weight.

Example:

upstream u {
    random [two [least_conn]];

    server 127.0.0.1:8080;
    server 127.0.0.1:8081;
    server 127.0.0.1:8082;
    server 127.0.0.1:8083;
}

The module allows passing requests to upstream gRPC servers.
The module is built by default as long as HTTP/2 support is compiled in.
Example configuration:

    grpc_pass 127.0.0.1:9000;

Alternatively, the "grpc://" scheme can be used:

    grpc_pass grpc://127.0.0.1:9000;

Keepalive support is available via the upstream keepalive module.  Note
that keepalive connections won't currently work with grpc-go as it fails
to handle SETTINGS_HEADER_TABLE_SIZE.

To use with SSL:

    grpc_pass grpcs://127.0.0.1:9000;

SSL connections use ALPN "h2" when available.  At least grpc-go works fine
without ALPN, so if ALPN is not available we just establish a connection
without it.

Tested with grpc-c++ and grpc-go.

Instead of turning off some randomly selected http modules
when --without-http is specified, just don't process the
whole http modules section.

IPv6 now compiled-in automatically if support is found.  If there is a need
to disable it for some reason, --with-cc-opt="-DNGX_HAVE_INET6=0" can be used
for this.

The ssl_preread module extracts information from the SSL Client Hello message
without terminating SSL.  Currently, only $ssl_preread_server_name variable
is supported, which contains server name from the SNI extension.