BrokerStore<->Zeek table - introdude &backend attribute

The &backend attribute allows for a much more convenient way of
interacting with brokerstores. One does not need to create a broker
store anymore - instead all of this is done internally.

The current state of this partially works. This should work fine for
persistence - but clones are currently not yet correctly attached.

src/Val.h

@@ -1030,6 +1030,8 @@ public:
 	// on zeek::RecordTypes.
 	static void DoneParsing();
 
+	void SetBrokerStore(const std::string& store) { broker_store = store; }
+
 protected:
 	void Init(IntrusivePtr<zeek::TableType> t);