Include repo etags in transactions

[HebaruSan]

Problem

In the course of investigating why available modules were missing in #3699, users shared their registries, which revealed that the server etags (see #2682) were getting out of sync with the available modules; users were ending up with old module lists but up-to-date etags. This meant that if they tried to Refresh immediately, it would say the list was already up to date, even though it wasn't.

Cause

I found this very confusing for quite a long while. That data is all part of the Registry object; shouldn't it all be saved to registry.json together or not at all? No! There is one specific scenario in which they can get out of sync:

1. Metadata retrieved from network successfully
2. Etags updated
3. Module list updated
4. Exception thrown while still inside transaction (any exception would cause this, but in practice it was ReinstallModuleKraken from Replace repo-reinst with kraken, handle in UIs #3344)
5. Transaction handling reverts to the most recently saved copy of the registry, which happened at the start of step 3, after the etags were updated but before the modules were updated

Changes

In #3828 we eliminated this exception, so this is already functionally fixed, but architecturally it's still important to make the data objects behave properly.

Now the etags are saved by a new Registry.SetETags function, which calls EnlistWithTransaction first, which will save off a copy of the registry with the original etags. This will ensure that if the registry is reverted in a transaction, the etags will accurately reflect the state of the available modules list.

[techman83]

That's quite an edge case, good find! LGTM