Improve Citus table size functions

Users currently have 2 sets of size functions:
- pg_*_size
- citus_*_size

The PostgreSQL functions give incorrect sizes for distributed tables & indexes on distributed tables. The Citus functions give errors for non-distributed tables & indexes.

We should fix the Citus size functions to handle both local & distributed tables as well as indexes.

Start by tracing the pg_*_size and citus_*_size functions and compare their behavior for local and distributed tables and indexes. Done means the Citus size functions return correct sizes for both table types and both index types without errors.

Written by the indexing model from the issue text.