Fixes#363
This change modifies the error message given when Citus is attempted
to be loaded other than shared_preload_libraries. Explanations have been
extended with that shared_preload_parameters parameter is in
postgresql.conf and citus should be at the beginning.
All citusdb references in
- extension, binary names
- file headers
- all configuration name prefixes
- error/warning messages
- some functions names
- regression tests
are changed to be citus.