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)

syntax: glob
*.pyc
*.pyv
*.swp
*.swo
tags
twistd.log
twistd.pid
bridge_constructor/generated
_trial_temp/
sat.egg-info
*.un~
dist
MANIFEST
build
ctags_links/
Session.vim
.build
.pytest_cache