* Added missing deferred cleanup directive to sub-test functions
why:
Rocksdb keeps the files locked for a short while leading to errors. This
was previously solved my using different db sub-directories
* Provide vertex deep-copy function globally.
why:
is just handy
* Avoid unnecessary vertex caching when merging proof nodes
also:
Run all merge tests on the rocksdb backend
Previously, proof node tests were run without backend
15cc9f962e