Issue9: support the concept of config generation ID

This is a proposed implementation of issue #9.

It generally implements the idea as discussed in comment lines
of https://bind10.isc.org/ticket/2918:
- introduced a generalized concept of "system reserved configuration items" (starting with an underscore)
- generation ID is one of (actually, currently the only) such reserved item
- generation IDs are managed by the config manager per module, and the config manager increments the IDs when it successfully updates a module's config
- reserved IDs are not part of the module spec, and so are hidden from users.  They cannot be modified via a user command.