[SPARK-19326] Speculated task attempts do not get launched in few scenarios

## What changes were proposed in this pull request?

Add a new listener event when a speculative task is created and notify it to ExecutorAllocationManager for requesting more executor.

## How was this patch tested?

- Added Unittests.
- For the test snippet in the jira:
val n = 100
val someRDD = sc.parallelize(1 to n, n)
someRDD.mapPartitionsWithIndex( (index: Int, it: Iterator[Int]) => {
if (index == 1) {
Thread.sleep(Long.MaxValue) // fake long running task(s)
}
it.toList.map(x => index + ", " + x).iterator
}).collect
With this code change, spark indicates 101 jobs are running (99 succeeded, 2 running and 1 is speculative job)

Author: Jane Wang <janewang@fb.com>

Closes #18492 from janewangfb/speculated_task_not_launched.