netmgr: libuv-based network manager

netmgr: libuv-based network manager

This is a replacement for the existing isc_socket and isc_socketmgr
implementation. It uses libuv for asynchronous network communication;
"networker" objects will be distributed across worker threads reading
incoming packets and sending them for processing.

UDP listener sockets automatically create an array of "child" sockets
so each worker can listen separately.

TCP sockets are shared amongst worker threads.

A TCPDNS socket is a wrapper around a TCP socket, which handles the
the two-byte length field at the beginning of DNS messages over TCP.

(Other wrapper socket types can be implemented in the future to handle
DNS over TLS, DNS over HTTPS, etc.)