Add metadata-only counterparts to ::database, ::keyspace, and ::table.
Apart from being metadata-only objects suitable for the coordinator,
the new types are also type-erased and so they can be mocked without
being linked to ::database and friends.
We use a single abstract class to mediate between data_dictionary
objects and the objects they represent (data_dictionary::impl).
This makes the data_dictionary objects very lightweight - they
only contain a pointer to the impl object (of which only one
needs to be instantiated), and a reference to the object that
is represented. This allows these objects to be easily passed
by value.
The abstraction is leaky: in one place it is outright breached
with data_dictionary::database::real_database() that returns
a ::database reference. This is used so that we can perform the
transition incrementally. Another place is that one of the
methods returns a secondary_index_manager, which in turn grants
access to the real objects. This will be addressed later, probably
by type erasing as well.
This patch only contains the interface, and no implementation. It
is somewhat messy since it mimics the years-old evolution of the
real objects, but maybe it will be easier to improve it now.
The new module will contain all schema related metadata, detached from
actual data access (provided by the database class). User types is the
first contents to be moved to the new module.
