Support for smart overwrite

[denis-stepanov]

Current db-djv-pg export / import -o behavior is to delete the existing object of the same name at the target and create it again. Actually, both database and files know SHA1 hash of the object, so a bit smarter behavior would be to delete + create only if SHA1 hashes differ.

[denis-stepanov]

Summary of implementation:

• default behavior does not change (no overwrite);
• -o option now checks SHA1 of source and target and refuses overwrite if they match. Previous behavior was unconditional overwrite;
• new -O option implements unconditional overwrite.

These options are implemented on commands export, import and rename. The latter works in both DB and file modes.