This patch adds a DecompressChunk custom scan node, which will be
used when querying hypertables with compressed chunks to transparently
decompress chunks.
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.
