[#1428] Delete multiple hosts by subnet/addr

Adjusted behavior of the del(subnet_id, addr) function in MySQL and
Postgres to delete multiple hosts having the same IPv4 or IPv6 address.
The MySQL schema had to be updated to use CASCADE action rather than
trigger to remove dependent options and IPv6 reservations.

src/lib/dhcpsrv/pgsql_host_data_source.h

@@ -108,7 +108,7 @@ public:
     /// violation
     virtual void add(const HostPtr& host);
 
-    /// @brief Attempts to delete a host by (subnet-id, address)
+    /// @brief Attempts to delete hosts by (subnet-id, address)
     ///
     /// This method supports both v4 and v6.
     ///