Database updates

* Improves performance by generating a standard digest (rather than the
slower hexdigest)
* Adds a #keys() method that returns a list of all primary keys
* Allows for the #array() method to handle JSON data
* Adds a new configuration flag, :checksum_on_insert, which allows
enabling or disabling the generation of a checksum value when a new
value is inserted (depends on the checksum_key property; default: true)
* Improves the #include? and #store_blob() methods.

This commit also adds the missing "databases" property to the local.dat
database.