A bug in the SQL for getting the size of chunks would use the
TOAST size of the main/dummy table as the toast size for the
chunks rather than each chunks' own toast size.
Getting an approximate row count for a hypertable involves getting
estimates for all of its chunks rather than just looking up a
single value in the catalog tables. This PR provides a convenience
function for doing the JOINs/summing.
The hypertable, chunk, and index size functions are
now split into main function and a corresponding ´pretty´
function. In chunk_relation_size_pretty() the ranges are
now converted into a human readable form when they are time types.
