Implement TCP manual read timer control functionality

Implement TCP manual read timer control functionality

This commit adds a manual TCP read timer control mode which is
supposed to override automatic resetting of the timer when any data is
received. That can be accomplished by
`isc__nmhandle_set_manual_timer()`.

This functionality is supposed to be used by multilevel networking
transports which require finer grained control over the read
timer (TLS Stream, DoH).

The commit is essentially an implementation of the functionality from
newer versions of BIND.