OPENDJ-2616 Support protection of pluggable backend data at rest

Allow for encryption of entries in ID2Entry, via a new backend option.
Encryption reuses the existing Cryptomanager for generating and maintaining keys.
Besides the entry themselves, EntryIDSets are also encrypted in substring indexes, with the same cipher as for ID2Entry. A new format is introduced to distinguish clear text vs. cipher text values: cipher text starts with a zero, the only
value not used by standard encoding.
Equality indexes have their keys reduced to a 6 bytes hash, using SHA-1.
Substring indexes encrypt with the same cipher transformation as ID2Entry their values instead.