Crash loop if node restarts during insertion of pruning-point UTXO Set

[svarogg]

This is what happens:

1. Node downloads pruningPoint utxo-set.
2. While importing the utxo-set node restarts
3. The importing of utxo-set resumes, but pruningPoint's ValidateAndInsertBlock will never be called.

This should probably be fixed while implementing #1734