Rename omnisci to heavydb

[pearu]

With #447, rbc supports two brands of heavydb:

• heavyai: version 6.0 and up
• omnisci: versions 5.x and older

In long term (1-2 years), omnisci brand is going to be deprecated and unsupported.

In short term, we'll need to switch to using heavyai brand name instead of omnisci brand name while still supporting the omniscidb 5.7 and up.

The switch means renaming heavydb related directories and file names as follows [this is a suggestion to be discussed]:

rbc/omniscidb.py -> rbc/heavydb.py
rbc/omnisci_backend/ - rbc/heavyai/
rbc/omnisci_backend/omnisci_xyz.py -> rbc/heavyai/xyz.py
rbc/tests/test_omnisci_xyz.py -> rbc/tests/heavyai/test_xyz.py
rbc/externals/omniscidb.py -> rbc/externals/heavydb.py

This issue will be actionable after heavydb 6.0 is released and heavydb conda packages will be available.