Wipe out record content on record delete

[lvca]

The delete of a record doesn't reset the content but just moves bytes around. This was mainly for efficiency. It would be cool to have a setting (enabled by default) to wipe out the content when deleting.

PROS:

• security: deleted content is not readable, not even from the raw file
• compression: having a sequence of 0 means the database can be more compressed at backup

CONS:

• a little bit slower on delete because of the additional write operation