Prevent termination races of Worker

[qwwdfsad]

Fixes #3578

[dkhalanskyjb]

Even if we decide to use #3585, let's still use this change. It prevents IllegalStateException in the case when the worker is terminated before ever entering runBlocking.

[dkhalanskyjb]

Now that I think about it, it doesn't look like IllegalStateException can really happen anymore.

    @Test
    fun testRunBlockingInAlreadyTerminatedWorker() {
        val finished = Channel<Unit>()
        Worker.start().apply {
            executeAfter(1_000_000) {
                runBlocking {
                    finished.send(Unit)
                }
            }
        }.requestTermination()
        runBlocking {
            finished.receive()
        }
    }

It passes with no problems. When the worker starts runBlocking, it's obviously doing some work, and that's enough to prevent it from terminating. So, it looks like the issue has been mitigated already.