Support for Window functions without partitions and with unbounded windows

[wmalpica]

This Issue assumes that PR #1426

has been merged.

The design for this feature can be found here:

https://docs.google.com/document/d/1DQMEps8Wgx757AS1GnDqGdeJiW88C7mLSLGOD_aaonQ/edit?usp=sharing