Evaluate renaming pg_dist_partition and related "partition" names

We started out by using the terminology "partition" to reflect our use of distributed tables. We then switched to "distribute by" in CitusDB to differentiate between partitioned and distributed tables.

I talked to Samay, and we're now using "distribution" instead of "partition" in our docs. We made this change because Postgres already used the term "partition" for machine local partitions; and Greenplum and Netezza have the concept of "distribution" to avoid this confusion.
1. We still have the term "partition" lingering in our system catalog tables. We also use it often in metadata accessor functions.
2. We further use the term "partition" during logical and physical planning.

We should consider fixing our terms at least for 1.