pj_ell_set now forwards the thread context (#198)

Reduced lock contest in pj_get_default_ctx. Mutex will no longer get hit after the default context has been initialized

multistresstest now runs under Windows (previously posix-only)
* Now reports an error if projection initialization fails with the default context but succeeds with a thread context (when reinit_every_iteration)
* Stress test runs substantially faster when reinit_every_iteration (due to less contest for the lock in pj_get_default_ctx)