TimestampProvider refactoring

Removed a method from this interface and its implementations, edited
javadoc on its methods
Replace some currentTimeMillis calls w/ Stopwatch
Tweak embedded store CF creation

Adding some logging and diagnostic code to help debug a racy
intermittent exception that shows up test.log as:

java.lang.RuntimeException: java.lang.IllegalArgumentException: Unknown table/cf pair (InternalCassandraEmbeddedKeyColumnValueTest.testStore1)
        at org.apache.cassandra.service.StorageProxy$DroppableRunnable.run(StorageProxy.java:1582)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.IllegalArgumentException: Unknown table/cf pair (InternalCassandraEmbeddedKeyColumnValueTest.testStore1)
        at org.apache.cassandra.db.Table.getColumnFamilyStore(Table.java:166)
        at org.apache.cassandra.db.Table.getRow(Table.java:354)
        at org.apache.cassandra.db.SliceFromReadCommand.getRow(SliceFromReadCommand.java:70)
        at org.apache.cassandra.service.StorageProxy$LocalReadRunnable.runMayThrow(StorageProxy.java:1052)
        at org.apache.cassandra.service.StorageProxy$DroppableRunnable.run(StorageProxy.java:1578)
        ... 3 more

And shows up in the surefire log as a read timeout, typically
immediately following a clopen() call but not necessarily.