ensure transaction states are changed only once

We want to keep track of the state of a transaction overall to base
future decisions on it, but as a pre-requirement we have to make sure
that a transaction isn't commited twice (which happened if the download
of InRelease failed and Release takes over).

It also happened to create empty commits after a transaction was already
aborted in cases in which the Release files were rejected.

This isn't effecting security at the moment, but to ensure this isn't
happening again and can never be bad a bunch of fatal error messages are
added to make regressions on this front visible.

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