When GET /v2/keys was orignally added in b263f47, prekeys were stored in
Postgres, with a user's unsigned and signed keys together in the same table.
Therefore GET /v2/keys subtracted one from the count returned by storage.
In d4d9403, we changed to a different storage schema, with unsigned prekeys in
one DynamoDB table and unsigned prekeys in the accounts Dynamo table.
Unfortunately, GET /v2/keys was not changed to stop subtracting one from the
count of prekeys in the keys table at the same time. This commit fixes that.
391b070cff