A transactional map

Adds the notion of isolated transactions to a map.

All operations defined on the cut down map interface net.pokle.dbtb.Database can be commited (or rolledback) in isolation to any changes made in other transactions. You can also chain transactions ala patchsets in git or mercurial.

Here's an example of what's possible: (From DatabaseSampleTest)

    // A base database
    HashMapDatabase<String, String> base = new HashMapDatabase<String, String>();
    // And its un-commited transaction
    Transaction<String, String> child = new Transaction<String, String>(base);

    // A modification to the base...
    base.put("The meaning of life", "0");

    // ... is visible to both the base and child transactions
    assertEquals("0", base.get("The meaning of life"));
    assertEquals("0", child.get("The meaning of life"));

    // A modification of the child transaction...
    child.put("The meaning of life", "42");

    // ... is only visible in the child transaction
    assertEquals("42", child.get("The meaning of life"));
    assertEquals("0", base.get("The meaning of life"));

    // You can commit a child transaction to its parent
    child.commit();
    assertEquals("42", child.get("The meaning of life"));
    assertEquals("42", base.get("The meaning of life"));