requireSqlCipherKey

Checks that key can encrypt a database on this platform. On iOS, it has to be SQLCipher's raw-key form with the salt, x'<64 hex digits key><32 hex digits salt>', as SecurityHelper creates it there from the key and salt it stores together. With the readable header, SQLCipher has no place to keep a salt of its own: it would encrypt a new database with a random one, and lose it.