Inline mergePair to look more like the narrow dep branch.

No functionality changes, I think this is just more consistent
given mergePair isn't called multiple times/recursive.

Also added a comment to explain the usual case of having two parent RDDs.
Handle Terminated to avoid endless DeathPactExceptions.

Credit to Roland Kuhn, Akka's tech lead, for pointing out this
various obvious fix, but StandaloneExecutorBackend.preStart's
catch block would never (ever) get hit, because all of the
operation's in preStart are async.

So, the System.exit in the catch block was skipped, and instead
Akka was sending Terminated messages which, since we didn't
handle, it turned into DeathPactException, which started
a postRestart/preStart infinite loop.
Merge branch 'master' into fixdeathpactexception

Conflicts:
	core/src/main/scala/spark/deploy/worker/Worker.scala
Merge pull request #451 from stephenh/fixdeathpactexception

Handle Terminated to avoid endless DeathPactExceptions.
Merge pull request #450 from stephenh/inlinemergepair

Inline mergePair to look more like the narrow dep branch.