There was a race condition in vtkDataEncoder when terminating all worker
threads. There was a potential that worker threads would terminate just before
the master thread started to wait for them to terminate. This patch addresses
that issue by ensure that no threads can terminate until the master thread
starts to wait for them.

Change-Id: I7c9b7571e03b7d19a30329403d5c4dbfc3c2ad3e