#127 genericize abstract StoreSPITest
 #127 fix incorrect getKeyType() calls
Merge pull request #129 from lorban/issue-127

Issue #127 StoreSPITest should use type parameters
draft of Expiration and impl in OnHeapStore
fix two arg remove() and replace(...) methods while operating on
present but expired entries
much cleaner implementation of expiration using underlying CHM compute() methods
allow Durations in any time unit
javadoc
test for OnHeapStore that includes expiration paths
unit tests and fix to Duration.hashCode()
move calls to TimeSource.getTimeMillis() inside functions since the
timestamp isn't needed unless the function is actually invoked
remove commented out code