ACCUMULO-2419 Reimplement SimpleTimer using executors

The SimpleTimer class is changed to use Java's executor service for thread pools, rather
than java.util.Timer. This allows Accumulo to run SimpleTimer threads using more than
one thread, for systems that can handle the load.

This change also transitions a wide variety of SimpleTimer users to use the new form of the
SimpleTimer.getInstance() method.
Revert "ACCUMULO-2490 Make Pair API more generics-friendly"

This reverts commit 053e83fc4225969efda31ee4957651640cc7ac9e.
ACCUMULO-2492 add time for zookeeper propogation
Merge branch '1.6.0-SNAPSHOT'
ACCUMULO-2490 Slight improvement to fromEntry w/test