The full user-defined structure of the database (keyspaces,
tables, user-defined types, and similar metadata, often known
as the schema in other databases) is needed by much of the
front-end code. But in Scylla it is deeply intertwined with the
replica data management code - ::database, ::keyspace, and
::table. Not only does the front-end not need data access, it
cannot get correct data via these objects since they represent
just one replica out of many.
This dual-role is a frequent cause of recompilations. It was solved
to some degree by forward declarations, but there is still a lot
of incidental dependencies.
To solve this, we introduce a data_dictionary module (and
namespace) to exclusively deal with greater schema metadata.
It is an interface, with a backing implementation by the existing code,
so it doesn't add a new source of truth. The plan is to allow mock
implementations for testing as well.
Test: unit (dev, release, debug).
Closes#9783
* github.com:scylladb/scylla:
cql3, related: switch to data_dictionary
test: cql_test_env: provide access to data_dictionary
storage_proxy: provide access to data_dictionary
database: implement data_dictionary interface
data_dictionary: add database/keyspace/table objects
data_dictionary: move keyspace_metadata to data_dictionary
data_dictionary: move user_types_metadata to new module data_dictionary
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