Some module reloading scenarios such as that which occurs
within mod_wsgi mean that an existing module-level enginefacade
is already in the "started" state, however initialization
routines from the calling application may still attempt to
call the ``.configure`` method. Add a new flag is_started
to both _TransactionContextManager and _TransactionFactory so
that calling code can check for this state ahead of time;
additionally, promote the TypeError raised to a specific subclass
enginefacade.AlreadyStartedError to allow for better optimistic
schemes.
Change-Id: I2f5a9e35c2fae0c28b78beef3dcd2c4794362766
References: I704196711d30c1124e713ac31111a8ea6fa2f1ba
5b36e169ea