make 500s change priorities quicker

it's not acceptable for a constantly failing provider to take 50 minutes
to drain (5x reducing priority by 10). But similarly, we need _some_
delay, or a handful of concurrent failures will completely turn off a
provider, rendering the whole excercise kinda pointless. Setting the
delay before it tries to reduce priority again to one minute is nice
because it means that if one request times out and returns 502, then any
other requests that are in flight at that time will time out before the
one minute is up and not switch, but any requests made after the switch
that take sixty seconds to time out will affect it.

app/delivery/send_to_providers.py

@@ -1,6 +1,6 @@
 import random
 from urllib import parse
-from datetime import datetime
+from datetime import datetime, timedelta
 
 from flask import current_app
 from notifications_utils.recipients import (
@@ -66,7 +66,7 @@ def send_sms_to_provider(notification):
             except Exception as e:
                 notification.billable_units = template.fragment_count
                 dao_update_notification(notification)
-                dao_reduce_sms_provider_priority(provider.get_name())
+                dao_reduce_sms_provider_priority(provider.get_name(), time_threshold=timedelta(minutes=1))
                 raise e
             else:
                 notification.billable_units = template.fragment_count