gzip csv files don't work with our custom sql generation

[cpcloud]

there are a few options here:

• error out before the database tries to read anything if there isn't a way to do this natively
  • error out on everything except postgres + UNIX OSs (postgres has a PROGRAM clause in its COPY command that allows use of an external program to e.g., gzcat the file in)
• do the above for postgres, and for everything else enforce this path csv -> chunks(temp(csv)) -> sa.Table
  • to enforce this without adding a bunch of conditionals, it might make sense to have a GZip metatype

cc @llllllllll @ltransom