[X] Explicitly close all snapshot iterators on snapshot release. Not closing iterators doesn't appear to cause a memory or a space leak on LevelDB and Pebble - this PR is meant to avoid any future issue with other types of DBs
[X] Fix race condition in swtichable module, which might cause opening extra iterator on snapshot switching (also doesn't seem to matter on LevelDB and Pebble)
This PR attemps to help resolving issue https://github.com/unicornultrafoundation/go-u2u/issues/9: