If you have a DAG with `max_active_runs=1` and it is running some tasks, and then you manually trigger another run of this DAG, both runs end up not scheduling anything and you see this message over and over in the logs:

```
[2020-10-16 14:00:34,118]  2437572 {{airflow.jobs.scheduler_job.SchedulerJob scheduler_job.py:1657}} INFO - DAG scenario1_case2_01_1 already has 1 active runs, not queuing any more tasks
```

A longer term fix for this would be to introduce a "queued" state for DagRuns, and then when manually creating dag runs set it to queued, and only have the scheduler set DagRuns to running.

That may or may not be less work than fixing the current mechanism.
airflow/jobs/scheduler_job.py
from sqlalchemy.orm import load_only, selectinload
from sqlalchemy.orm.session import Session, make_transient
from airflow import models, settings
from airflow.configuration import conf
from airflow.exceptions import AirflowException, TaskNotFound
from airflow.executors.executor_loader import UNPICKLEABLE_EXECUTORS
from airflow.jobs.base_job import BaseJob
from airflow.models import DAG, DagModel, SlaMiss, errors
from airflow.models.dagbag import DagBag
from airflow.models.dagrun import DagRun
from airflow.models.serialized_dag import SerializedDagModel
from airflow.models.taskinstance import SimpleTaskInstance, TaskInstanceKey
from airflow.stats import Stats
from airflow.ti_deps.dependencies_states import EXECUTION_STATES
from airflow.utils import timezone
from airflow.utils.callback_requests import (
    CallbackRequest, DagCallbackRequest, SlaCallbackRequest, TaskCallbackRequest,
)
from airflow.utils.dag_processing import AbstractDagFileProcessorProcess, DagFileProcessorAgent
from airflow.utils.email import get_email_address_list, send_email
from airflow.utils.log.logging_mixin import LoggingMixin, StreamLogWriter, set_context
from airflow.utils.mixins import MultiprocessingStartMethodMixin
from airflow.utils.session import create_session, provide_session
from airflow.utils.sqlalchemy import is_lock_not_available_error, prohibit_commit, skip_locked, with_row_locks
from airflow.utils.state import State
from airflow.utils.types import DagRunType
TI = models.TaskInstance
DR = models.DagRun
DM = models.DagModel
class DagFileProcessorProcess(AbstractDagFileProcessorProcess, LoggingMixin, MultiprocessingStartMethodMixin):