Code cleanup, more on variables capabilities:
- Unify all the variable name constants into a single file, VariableConstants.
- Add a new catalog object, VariableConfig, which will replace GlobalConfig and hold
  variable value metadata, scoping, etc.
- Use the known variable objects where necessary.
- Arrange to initialize variable subsystem on system startup, remove reading of xml files.
- Arrange to find session variable stmt off of new variable subsystem
- Arrange to hook up cache segments with their corresponding variables.
- Arrange for error messages from value metadata classes to be more informative.
- Arrange for nullable/non-nullable variables.
- Generalize the VariableAccessor framework to handle user/session/globals in the same way.

Many tests still do not work, as not all of the variables have been migrated yet.
Also, values for result sets seem to be incorrectly formatted.