Xodus
Based on your requirements, I think JetBrains Xodus is a very choice of database. It has three types of database- a key-value storage, an entity-store and a database for files and streams.
#1: It is very easy to start using. If you are using maven just add one or two dependencies. Or you can also import .jar files directly into your project.
No need to create any server. Just create an instant of Environment and you are ready to go. And you can give it your folder name to store data.
final Environment env = Environments.newInstance("/Users/me/.myAppData");
#2: Instead of managing tables you can just store entities. It is very similar to relational tables, but a lot more easier. Take a look here to know how to do it.
final Entity user = txn.newEntity("User");
user.setProperty("login", loginName);
user.setProperty("fullName", fullName);
user.setProperty("email", email);
final String salt = MessageDigestUtil.sha256(Double.valueOf(Math.random()).toString());
user.setProperty("salt", salt);
user.setProperty("password", MessageDigestUtil.sha256(salt + password));
#3: More lightweight than any other non-relational database avaiable. See their benchmark test.
