lib/trie: prepare trie nodes for mutation only when mutation is guaranteed

[qdm12]

Preparing a trie node for mutation means:

1. Report the hash of the existing trie node as a deleted hash for the online pruner
2. Deep copy the trie node and update the trie 'generation' (bumped on every trie snapshot) on the new node copy

There has already been some work to reduce the number of trie node mutation preparations. However, this was more for performance reasons (CPU and memory usage) by reducing the node deep copying when not needed.

Working on #2835 I realized about 1. mentioned above, and checked our trie code again for these mutation preparations.
Some of them are appropriate, but some happen although mutation isn't guaranteed. In this case, it essentially means our online pruner would just throw out valid and currently in use node hashes from the database. We didn't spot this so far since we don't run the online pruning really.

To fix this, we should change those few occurrences to occur ONLY when mutation is GUARANTEED.

[qdm12]

Done in #2834