DERBY-6169: Reduce visibility of classes and methods under impl/sql

- Reduce visibility of some classes, fields and constructors under
  impl/sql/execute

- Expose InternalTriggerExecutionContext.getAutoincrementValue()
  through the interface so that callers don't have to cast the
  context to the internal implementation type

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1467201 13f79535-47bb-0310-9956-ffa450edef68