Add isc_nmhandle_setwritetimeout() function

Add isc_nmhandle_setwritetimeout() function

In some situations (unit test and forthcoming XFR timeouts MR), we need
to modify the write timeout independently of the read timeout.  Add a
isc_nmhandle_setwritetimeout() function that could be called before
isc_nm_send() to specify a custom write timeout interval.

(cherry picked from commit a89d9e0fa68b8d915c6a1c416543dd157d8b0b5a)