Pause IBD during AssumeUTXO snapshot load

[Sjors]

Please describe the feature you'd like to see added.

The loadtxoutset RPC call should cause IBD to pause until the snapshot is loaded and the snapshot chain is activated.

Is your feature related to a problem, if so please describe it.

When I start a fresh node and load a mainnet shapshot (#28553) any progress is completely buried in the log.

More importantly, I get the strong impression, though haven't properly tested this, that the IBD process slows down the snapshot load. This makes sense given the amount of I/O involved. It's definitely slower than the process of creating the snapshot.

On my AMD Ryzen 7950x machine the IBD reaches block 300,000 while the snapshot has only loaded 71.2% (where is triggers a cache flush). By the time 100% is loaded IBD is at 324,000. When the snapshot chainstate is saved IBD reaches 362,000.

Describe the solution you'd like

Pause IBD until the snapshot is loaded and the snapshot chain is activated.

Describe any alternatives you've considered

1. Do nothing
2. Keep IBD going but at low priority (how???)

Please leave any additional context

No response