After the TCPDNS refactoring the initial and idle timers were broken and
only the tcp-initial-timeout was always applied on the whole TCP
connection.

This broke any TCP connection that took longer than tcp-initial-timeout,
most often this would affect large zone AXFRs.

This commit changes the timeout logic in this way:

  * On TCP connection accept the tcp-initial-timeout is applied
    and the timer is started
  * When we are processing and/or sending any DNS message the timer is
    stopped
  * When we stop processing all DNS messages, the tcp-idle-timeout
    is applied and the timer is started again