Encrypt with MK pages 1-4 of system tablespace

Keyring encryption based on MariaDB implementation does not encrypt  FSP_TRX_SYS_PAGE_NO as it contains double write buffer header. Ours MK system tablespace encryption on the other hand also does not encrypt pages 1-4. Seems it should be safe to encrypt those pages. There should not be backward incompatibility if we start to encrypt those pages now as encryption information can be set for not encrypted page. The page's decryption function looks at the page's header and if it sees that the page is not encrypted it returns and leaves the page intact.