This is the first ticket for the production implementation of session windows. This ticket includes all the major changes in the window classes to support session windows. This ticket does not include any changes in the planner.

• Introduce session window mode
• Add separate data structure for tracking session windows
• Add session window assignment, merge, and close logic
• Amend window close and merge signal
• Add merge support in group, sort, and infinite limit
• Add unit tests. The tests should cover the below
  • partition logic
  • merge logic
  • watermark handling
  • validate expected results
  • See this file for some example unit tests: https://github.com/10gen/mongo/blob/master/src/mongo/db/modules/enterprise/src/streams/exec/tests/window_aware_operator_test.cpp
    color: Color value is invalid

Don't worry about closeOn behavior in this ticket.