- Avoid fetching next seq from tree on every op store
- Misc cleanups

- Eager async fetch of entries to import
- Drop old tree history (improves situation but it's not a permanent solution, more investigation required) with async iavl tree pruning
- Skip executing import on proposal preparation
- Effectively enforce timeouts through proper context propagation and checking
- Iterate mutable tree with correct iterator when obtaining latest sequence value
- Avoid repeated lock acquisition when importing multiple operations