[DB migrations] Added downgrade support and AlembicUtil Tests

[quaark]

When initializing the database:

• On db initialization a backup of the DB is made before the upgrade to the latest schema revision, with its previous alembic revision as its name (Any new backup overrides the old backup if it is the same revision)
• If the current DB revision doesn't exist in the known revision history, this means the DB is too new for the code, and we will try to fall back to the backup of the latest know revision.
• If no such backup exists - raise a RuntimeError and fail initializing the DB.

In addition - completely unit-tested the AlembicUtil

[Hedingber]

Think of this scenario:
MLRun is in version X -> we're installing version X+1, migration succeeds (so backup created at this point) -> user uses this X+1 (so data is added) -> we're installing version X+2, migration failed -> we're installing X+1 again -> it uses the backup, but we lost a lot of the data.
I think you should backup the DB before the migration (before the action that might fail and you'll want to revert)

[Hedingber]

Looks very good.
Test coverage - 💯 ❤️
one comment inside, also please update the PR description