Removed User, Group, Base request priorities

Removes handling of
    ReqMgrGroupBasePriority
    ReqMgrRequestBasePriority
    ReqMgrRequestorBasePriority

Oracle tables values are still there and Oracle database wasn't and won't be
altered, but above values are not used anymore in the code. Only a single
inject and request parameter 'RequestPriority' is now used. It's value is
updated absolutely, not by offset as it probably was the previous intention
(as agreed via IM by @efajardo).

Value of reqmgr_assignment:priority_modifier from database definition - also
ignored, as well as a bunch of priority related database table columns, as
said above, except for reqmgr_request:request_priority value. This always
agrees with CouchDB "RequestPriority" document field.

unittests and reqmgr.py --allTests OK

Fixes #4465