TCP: short writes handling

This changeset enables handling of short writes on TCP connections.
Also, the default send/receive buffer sizes for server TCP sockets is
increased to make fast-path processing more likely.

Formerly, Knot DNS running on FreeBSD 10 failed to answer large AXFR
queries. The default send buffer is too small (32768 bytes) to fit
the whole message. Since the sockets are non-blocking, a short write
happened and the TCP handler code terminated the connection.

See merge request !353