Mode 1708 - Implemented queryable option for connectors and internal nodes

Added a new document flag "queryable" which can be used both by connectors and internal nodes to make themselves non-queryable (by default, all nodes are being indexed).

For the internal nodes, the _CachedNode_ hierarchy was updated to be able to _set/get_ the value for this flag and also for _SessionNodes_ to have "transient" queryable state.

For connectors, a new configuration option: _queryable_ was added to the external sources, which allows a connector to turn off querying and indexing for its entire content. Also, a connector can use the _DocumentWriter_ to mark only a certain document as not-queryable.