OAK-1535: ClockTest.testClockDrift failures

Increase accuracy of the accurate clock.
Introduce a configurable random noise element to the simple clock.
Remove the accidentally committed &~0xfL test modifier from System.currentTimeMillis()

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1577177 13f79535-47bb-0310-9956-ffa450edef68
OAK-1488 - disable test for DOCUMENT_JDBC again

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1577183 13f79535-47bb-0310-9956-ffa450edef68