netmgr is missing TCP send timeout

When the uv_write() fills up the TCP send buffers (around 208k) on Linux because the other side is not reading, the TCP connection will be kept open indefinitely.

We need to add a "idle" timer around the uv_write(), so we can bail out early.

This also affects #1897 (closed) - the XFR timeout works fine, but the connection gets stuck as we have no way of terminating the pending uv_write()s on the stuck transfer.

FTR This is not CVE/ASN worthy because there's even a simpler way to keep the TCP connection open indefinitely - just keep sending and reading the DNS queries/responses, but keeping this confidential for the time being.