* Add errcheck linter

* Add config for 'fast' linters

* Fix errcheck lint stuff

* Fix errcheck lint

* Make txn *sql.Tx arguments optional everywhere using a utility function

* Clarify that if the txn is nil the stmt will run outside a transaction

* Add input API for adding invites to the roomserver.

This API handles invites received over federation that occur outside of a room.

* Add some docstring for withTransaction

* Use a nicer pattern for wrapping transactions

* Fix MembershipUpdater method to not commit the transaction before returning it

* Use the Transaction interface from common

* Add helper method for preparing lists of statements.