add dao_reduce_sms_provider_priority function

retrive the sms providers from the DB, and decrease the chosen
provider's priority by 10, while increasing the other by 10.

add a check in to ensure we never decrease below 0 or increase above 100
- this is per provider, we don't check that the two add up to 100 or
  anything. If the values are outside of this range (eg: set via the UI)
then they'll probably* fix themselves at some point - we've added tests
to document these cases.

Use with_for_update to ensure that the method can only run once at a
time - other invocations of the function will be held on that line until
the currently running one ends and commits the transaction. This doesn't
affect anyone doing things from the UI.

app/dao/provider_details_dao.py

@@ -22,13 +22,11 @@ def get_provider_details_by_identifier(identifier):
 
 
 def get_alternative_sms_provider(identifier):
-    alternate_provider = None
     if identifier == 'firetext':
-        alternate_provider = 'mmg'
+        return 'mmg'
     elif identifier == 'mmg':
-        alternate_provider = 'firetext'
-
-    return ProviderDetails.query.filter_by(identifier=alternate_provider).one()
+        return 'firetext'
+    raise ValueError('Unrecognised sms provider {}'.format(identifier))
 
 
 def get_current_provider(notification_type):
@@ -49,9 +47,23 @@ def dao_get_provider_versions(provider_id):
 
 
 @transactional
-def dao_toggle_sms_provider(identifier):
-    alternate_provider = get_alternative_sms_provider(identifier)
-    dao_switch_sms_provider_to_provider_with_identifier(alternate_provider.identifier)
+def dao_reduce_sms_provider_priority(identifier):
+    # get current priority of both providers
+    q = ProviderDetails.query.filter(
+        ProviderDetails.notification_type == 'sms',
+        ProviderDetails.active
+    ).with_for_update()
+
+    providers = {provider.identifier: provider for provider in q}
+    other = get_alternative_sms_provider(identifier)
+
+    # always keep values between 0 and 100
+    providers[identifier].priority = max(0, providers[identifier].priority - 10)
+    providers[other].priority = min(100, providers[other].priority + 10)
+
+
+def dao_toggle_sms_provider(*args, **kwargs):
+    raise NotImplementedError
 
 
 @transactional