Rename enums to lowercase

[robfrank]

In the code there are many enum types with uppercase names:

  enum DIRECTION {
    OUT, IN, BOTH
  }
  enum TRANSACTION_ISOLATION_LEVEL {
    READ_COMMITTED, REPEATABLE_READ
  }

These enums should be renamed to use lowercase.
These changes could impact our Java users, so it's probably better to ask the community what they think about these changes.