Add &default_insert attribute for tables

This is based on the discussion in zeek/zeek#2668. Using &default with tables
can be confusing as the default value is not inserted. The following example
prints an empty table at the end even new Service records was instantiated.

    type Service: record {
        occurrences: count &default=0;
        last_seen: time &default=network_time();
    };

    global services: table[string] of Service &default=Service();

    event zeek_init()
        {
        services["http"]$occurrences += 1;
        services["http"]$last_seen = network_time();

        print services;
        }

Changing above &default to &default_insert will insert the newly created
default value upon a missed lookup and act less surprising.

Other examples that caused confusion previously revolved around table of sets
 or table of vectors and `add` or `+=` not working as expected.

    tbl_of_vector["http"] += 1
    add tbl_of_set["http"][1];

src/Val.h

@@ -878,11 +878,13 @@ public:
 
 	/**
 	 * Finds an index in the table and returns its associated value or else
-	 * the &default value.
+	 * the &default or &default_insert value. If the &default_insert attribute
+	 * is set on the table, the returned value is also inserted into the table.
 	 * @param index  The index to lookup in the table.
 	 * @return  The value associated with the index.  If the index doesn't
-	 * exist, instead returns the &default value.  If there's no &default
-	 * attribute, then nullptr is still returned for nonexistent index.
+	 * exist, instead returns the &default or &default_insert.  If there's no
+	 * &default or &default_insert attribute, then nullptr is still returned
+	 * for nonexistent index.
 	 */
 	ValPtr FindOrDefault(const ValPtr& index);