pointcloud_formats needs to use a sequence to generate its IDs

[hobu]

The PDAL code that creates entries in pointcloud_formats is really dumb. In situations where multiple processes are creating pointcloud entries, it falls over. As a stop-gap, I have implemented some code to use a sequence on the pointcloud_formats table:

create sequence pointcloud_formats_pcid_sq;
alter sequence pointcloud_formats_pcid_sq owned by pointcloud_formats.pcid;
alter table pointcloud_formats alter column pcid set default nextval('pointcloud_formats_pcid_sq');

PDAL's writers.pgpointcloud will now use this sequence if it exists to generate the ID for the next pointcloud_formats entry. Otherwise it will do its silly MAX(id) approach. A more permanent fix is for pgpointcloud to add this sequence to the pointcloud_formats table when it is created.

See PDAL/PDAL#1101 for more detail