Check for jps in $JAVA_HOME/bin when not on $PATH

Addresses problem 1 of 2 in issue #538
titan.sh: kill Cassandra only after Rexster dies

`titan.sh stop` now waits up to 60 seconds by default (though this is
configurable through an environment variable) for Rexster to disappear
from the jps process table before attempting to kill Cassandra.  This
addresses issue 2 of 2 in #538.
Replacing AtomicLongs with Metrics Counters

Because Metric counters do not support the notion of set/reset/clear,
only increment and decrement, ExpirationStoreCache's metrics reset
methods are now at best approximate.  The reset methods may set their
respective counters back to zero, or they set them down to a value
slightly above zero.