Throttle reading from TCP if the sends are not getting through

Throttle reading from TCP if the sends are not getting through

When TCP client would not read the DNS message sent to them, the TCP
sends inside named would accumulate and cause degradation of the
service.  Throttle the reading from the TCP socket when we accumulate
enough DNS data to be sent.  Currently this is limited in a way that a
single largest possible DNS message can fit into the buffer.

(cherry picked from commit 26006f7b44474819fac2a76dc6cd6f69f0d76828)