[3971] Better handling errors when thread is stopped/started.

Also, improved some LFC unit tests.

src/bin/dhcp4/json_config_parser.cc

@@ -27,6 +27,7 @@
 #include <dhcpsrv/parsers/host_reservation_parser.h>
 #include <dhcpsrv/parsers/host_reservations_list_parser.h>
 #include <dhcpsrv/parsers/ifaces_config_parser.h>
+#include <dhcpsrv/timer_mgr.h>
 #include <config/command_mgr.h>
 #include <util/encode/hex.h>
 #include <util/strutil.h>
@@ -461,6 +462,9 @@ configureDhcp4Server(Dhcpv4Srv&, isc::data::ConstElementPtr config_set) {
     // so newly recreated configuration starts with first subnet-id equal 1.
     Subnet::resetSubnetID();
 
+    // Remove any existing timers.
+    TimerMgr::instance()->unregisterTimers();
+
     // Some of the values specified in the configuration depend on
     // other values. Typically, the values in the subnet4 structure
     // depend on the global values. Also, option values configuration