Dataset primary-key hash options

[ppKrauss]

The key column of dataset.big can be NULL, but ideal is to keep an primary-key even when it is long and complex. Adding _SHA1_, _MD5_ or _CRC32_ hashes as option is an issue for long-text keys, or when no key exists. Options:

• hash(key): when primary key exist and the option is used;
• hash(c::text): when the option is used but there are no primary key;
• hash(other_key): "overwrite" to a new primary key.

Ideal is to convert hash to base64 in order to optimize comparisons and reduce space.