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.

lib/isc/include/isc/netmgr.h

@@ -697,3 +697,6 @@ isc__nm_force_tid(int tid);
  * Force the thread ID to 'tid'. This is STRICTLY for use in unit
  * tests and should not be used in any production code.
  */
+
+void
+isc_nmhandle_setwritetimeout(isc_nmhandle_t *handle, uint64_t write_timeout);