Adds a new --jobs command-line argument to specify how many worker
connections you want. These worker connections should stick around
while processing table(s) in a single database. For each table,
parcel out the indexes to be built among these worker conns,
submitting each CREATE INDEX ... request using PQsendQuery() i.e.
in non-blocking fashion.
Most of this is still rather crude, in particular the
while (num_active_workers) ... loop in rebuild_indexes(), but
it seems to be working, so I'm committing here.
509e568c52