Switch from semaphores to message queue to share workload. (#305)

Using semaphores for work-sharing isn't ideal and brings some complexity and
inefficiencies (each worker process loops over the whole table array). Now
that we have introduced Sys V queues, use that mechanism instead to share
the workload between table-data COPY workers.

We still need to use a semaphore to create the lockfile and donefile so that
we can track progress, resume from interrupted operations, and discover if
all the partitions of a single table have been processed already. That said,
we don't have to deal with same-table concurrency using the semaphore and
files, the queue solves that for us.