DDLSHELL-22. Make HASH PREFIXED(2) the default key format.

If the user doesn't specify a key format, use a single hash-prefixed
string key component. The hash prefix is 2 bytes of the MD5 sum of
the key.

The previous default was to persist the 16-byte MD5 hash of the key,
and drop the key's plaintext entirely.

Signed-off-by: Shashir Reddy <shashir@wibidata.com>