compile_parallel.py: fix race condition with multiple proofs
compile_parallel.py: fix race condition with multiple proofs

when sending a proof query containing lots of earlier proofs,
we might get out of sync with coqtop.  synchronize by sending
a dummy Inductive definition and waiting for that definition
to be echoed back.

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl