Added retry for zookeeper_init per MESOS-1326.

Often during temporary DNS failovers or outages, we see slaves
aborting in zookeeper_init.

In many cases, the slave can restart within 10 seconds. Since
retrying zookeeper_init should be safe to do, this is an attempt
to minimize the number of unnecessary aborts in the slave.

Review: https://reviews.apache.org/r/21783
Reduced acknowledgment delay in the scheduler driver.

We currently 'defer' in order to acknowledge processed status
updates in the scheduler driver.

This is unfortunate because it adds unnecessary delay and
unnecessary retries, especially when the scheduler's queue
is backed up.

It is sufficient to use 'aborted' to see if we can send a status
update. Ideally there would be proper locking on this variable
rather than the use of 'volatile', but since we're mostly concerned
about the driver aborting on the same thread as the status update
was processed, I've left it as is.

Review: https://reviews.apache.org/r/21788