Improve Shard logging output

Improve Shard logging output

When debugging it's useful to see the shard name in the log output.

Also changed ShardIdentifier to cache the toString() output as the
class is immutable so we don't incur the overhead of building the
String for every log message.

Change-Id: Ic7ea9878eeab04ea9b43a25b7d4b2b190f79e607
Signed-off-by: tpantelis <tpanteli@brocade.com>