[bug] delay couch state initialization

The couch state checks if all user databases have the correct schema version on initialization, and may log errors and raise exceptions if that is not the case. Because we are currently using twistd web --wsgi, if those errors are logged too early the reactor may have not been started and the twistd logging facilities may not have been initialized.

This commit delays the state initialization until the reactor has been started, to make sure any errors raised and logged in that stage will actually reach the logfile.