Add index to Task.status

Summary:
We have monitoring and clean-up tasks that engage in table scans because they primarily care about
status; this is currently triggering our long-query warnings and creating noise.
Hopefully an index fixes that.
This is the same as 8c49ac5 but not concurrent.

Test Plan: None

Reviewers: naphat

Reviewed By: naphat

Subscribers: changesbot, anupc

Differential Revision: https://tails.corp.dropbox.com/D233891

migrations/versions/181adec926e2_add_status_index_to_task.py

@@ -0,0 +1,21 @@
+"""Add status index to task
+
+Revision ID: 181adec926e2
+Revises: 43397e521791
+Create Date: 2016-10-03 17:41:44.038137
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '181adec926e2'
+down_revision = '43397e521791'
+
+from alembic import op
+
+
+def upgrade():
+    op.create_index('idx_task_status', 'task', ['status'], unique=False)
+
+
+def downgrade():
+    op.drop_index('id_task_status', table_name='task')