Fixes race condition in PublishedSnapshotService and more...

[Shazwazza]

Fixes #5035

There's an issue in some rare cases where if multiple appdomains startup at once that the nucache files can get locked. This is mostly prevalent on Azure web apps when using the slot swap functionality.

Other things/issues:

• during slot swap the umbraco url gets corrupted because we blindly just accept the next domain request that comes in is the umbraco url
• a few TODOs and notes added
• publicizes the PublishedSnapshotServiceOptions so that devs on startup can choose to not have persisted cache files

[warrenbuckley]

Code looks fine, just remember we need to come back round to those TODOs @Shazwazza
Reporter/community have confirmed a fix, but I am off to test & sanity check, nothing blows up

[warrenbuckley]

Tested ZIP build and all works fine for me