Use insert .. on conflict for bulk import operations

[elexisvenator]

Updates the behaviour of BulkInsertMode.OverwriteExisting to use insert ... on conflict do update. This is one less db statement than before and a less prone to constraint issues.
Also updates the behaviour of BulkInsertMode.IgnoreDuplicates to use insert ... on conflict do nothing.

(Wasn't going to create this so soon but ran into an issue where spamming some seed data resulted in unique constraint violations when using OverwriteExisting)

Tests pass but I had to delete the checked in generated code files. I am not sure what the correct way to regenerate these files is - can this be added to the doco somewhere so I can fix the issue myself next time 🙇 .

[jeremydmiller]

I'm going to pull this down locally and try it first. Can't see any reason why this would fail offhand

[jeremydmiller]

This is a breaking change because it requires rebuilding all generated code. Hmm, think this might have to wait for Marten 8.

Which I do actually want to get to relatively soon-ish

[elexisvenator]

Hi @jeremydmiller,

I have given this another pass. Bulk inserting now supports conditional overwrites - when in BulkLoadMode.OverwriteExisting it is now possible to pass in a condition that must pass before a document is overwritten. This is very useful for bulk loading from external sources as it allows for safe out-of-order imports as long as documents have some sort of versioning column. Previously the only way to do this was to first query all the documents you wanted to import and drop the ones that were out of date. This way should be a lot faster.