Change account ids to snowflake ids (#15844)

Co-authored-by: Eugen Rochko <eugen@zeonfederated.com>

db/migrate/20210306164523_account_ids_to_timestamp_ids.rb

@@ -0,0 +1,17 @@
+class AccountIdsToTimestampIds < ActiveRecord::Migration[5.1]
+  def up
+    # Set up the accounts.id column to use our timestamp-based IDs.
+    safety_assured do
+      execute("ALTER TABLE accounts ALTER COLUMN id SET DEFAULT timestamp_id('accounts')")
+    end
+
+    # Make sure we have a sequence to use.
+    Mastodon::Snowflake.ensure_id_sequences_exist
+  end
+
+  def down
+    execute("LOCK accounts")
+    execute("SELECT setval('accounts_id_seq', (SELECT MAX(id) FROM accounts))")
+    execute("ALTER TABLE accounts ALTER COLUMN id SET DEFAULT nextval('accounts_id_seq')")
+  end
+end