Support for partial ordering/comparableKey in Java

[simerplaha]

On Scala side the KeyOrder type requires comparableKey which is used to index partially order keys. This is currently not available in Java. See test SwayDBPartialSetSpec to see how partial ordering is used.

Partial ordering basically allows fetching a key and value by giving it a partial key.

Suppose the type of key is

case class User(username: String, email: Option[String])

and a database entry exists for

User("user1", Some("email@email.com"))

then

db.getKey(User("user1", None))

would return the full key with the email Some("email@email.com") populated.

On Java side Comparator is used (See MapTest) which should be replaced with another interface JavaKeyOrder (similar to Scala's KeyOrder) to allow for comparableKey implementation.