feat: Block clio if migration is blocking (#1834)

Add:
- Block server if migration is blocking
- Initialise the migration related table when server starts against
empty DB

Add MigrationInspectorInterface. server uses inspector to check the
migrators status.

tests/unit/migration/MigrationManagerBaseTests.cpp

@@ -54,7 +54,6 @@ struct MigrationManagerBaseTest : public util::prometheus::WithMockPrometheus, p
     MigrationManagerBaseTest()
     {
         auto mockBackendPtr = backend_.operator std::shared_ptr<MockMigrationBackend>();
-        TestMigratorRegister const migratorRegister(mockBackendPtr);
         migrationManager = std::make_shared<TestCassandraMigrationManager>(mockBackendPtr, cfg.getObject("migration"));
     }
 };