let on_delete() return false to skip deletion

[edam]

When the on_delete() function returns false, the library skips the deletion of the row.

Returning true (or nil) causes the row to be deleted as normal, so this change has no effect on existing code.

Note that when an row is not deleted, it will ordinarily retain its position in the index and be a candidate for deletion on the next pass. Therefore, user should be warned that if they return false from on_delete(), then they must either:
a. modify the expiration field (to prolong the row from expiry),
b. delete the row themselves, or
a. expect another deletion attempt immediately.