Update comment relating to CASSANDRA-5566
HBase/Cassandra read op timestamp tweaks

This commit mostly affects read timestamps used by the Cassandra
embedded adapter, which could previously break reads on columns with
TTLs set when Titan was configured for a timestamp resolution other
than milliseconds.

All of the HBase and Cassandra backends should continue to retrieve
the most recent version of rows/columns, regardless of
TransactionHandleConfig.getStartTime().  The notion of start time is
generally not practical when using native HBase and Cassandra
timestamps, since Cassandra only shows the most recent column version
and HBase retains only between 1 and 3 most recent versions under its
default configuration (and may well be configured to retain only 1).

There's also some minor refactoring in DistributedStoreManager; I
renamed "Timestamp" to "MaskedTimestamp" in an attempt to highlight
this class's sole function, which is to encapsulate some bitmask
operations on otherwise ordinary timestamps.

There is more timestamp refactoring to come after this commit; this
just focuses on some delicate changes in the Cassandra embedded
adapter and refactoring in DistributedStoreManager.(Masked)Timestamp
and its usage sites.
Some Cassandra warning cleanups
Removing start time from TransactionHandleConfig

This currently has no robust mapping to either HBase or Cassandra, and
there's little use for it in the context of BDB.  It was unused
outside of wrapper classes and test mocks.
Make testGlob tolerate unclean target directory

HDFsToolsTest#testGlob no longer relies on JUnit test execution order
and FileSystem.globStatus(...) return array element order to pass.