Tolerate vector clock conflicts in the blueprint

Previously, namespace_repo_t would crash if there was ever a vector clock conflict in the blueprint. Now it silently ignores the conflict; while this isn't guaranteed to preserve availability, at least the server will stay alive so that the user can fix the problem.
Add regression test for issue 2738.
Don't generate new blueprints unnecessarily.

Previously, we would generate a new blueprint for every table after a table_create() operation, which could lead to a lot of spurious vector clock conflicts. With this commit, we only generate a new blueprint for the newly created table.