Modify SpacetimeDB to assert that current module's `__describe_module__` == `__describe_module__` derived from the system tables

[cloutiertyler]

Currently SpacetimeDB only checks:

• old module's __describe_module__ == new modules __describe_module__

This ticket proposes to modify SpacetimeDB that before a database is considered up and running, we should be asserting that the __describe_module__ matches a hypothetical __describe_module__ derived from the system table

If the ModuleDefs do match, then we start the database.

If the ModuleDefs do NOT match, then we can either panic if we can't upgrade to the new ModuleDef, or we could attempt an upgrade (probably with a warning since we are basically healing something that should never have happened).

We could also just choose to always panic on a mismatch.