increased PBKDF2 iteration count for v00 encryption from 10000 to 512000

This release increases the PBKDF2 iteration count for the additional password-based v00 encryption from 10.000 (which is the minimal NIST recommendation [1]) to 512.000 (which is even higher than the OWASP recommendation [2]). To be backwards compatible for now, password-protected secrets with the lower iteration count can be decrypted as well.

[1] https://pages.nist.gov/800-63-3/sp800-63b.html
[2] https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html