Replace custom parsing of order by and segment by lists
with the postgres parser. The segment by list is now
parsed in the same way as the GROUP BY clause and the
order by list in the same way as the ORDER BY clause.
Also fix default for nulls first/last to follow the PG
convention: LAST for ASC, FIRST for DESC.
This is useful, if some or all compressed columns are NULL.
The count reflects the number of uncompressed rows that are
in the compressed row. Stored as a 32-bit integer.
Add support for compress_chunks function.
This also adds support for compress_orderby and compress_segmentby
parameters in ALTER TABLE. These parameteres are used by the
compress_chunks function.
The parsing code will most likely be changed to use PG raw_parser
function.
