core (memory/sqla_mapping): fix some technical debt: a first Alembic revision file is joined to migrate existing databases to new mapping. This patch: - set `nullable=False` in various places - drop legacy `message_types` table, and use `Enum` instead - use naming convention, and get rid of anonymous constraint (this is the main reason why the revision file is so long)

You'll find install instructions in documentation (see doc/installation.rst).

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