Major untested Metrics refactoring

Made Metrics configuration transaction-centric.  Each graph
transaction can be configured to disable metrics, enable metrics, or
enable metrics with a custom metric name prefix.  The last option is
intended to let users segregate Metrics measurements generated by
Titan in whatever way they like -- for instance, by setting the
metrics prefix to the name of an application or the name of an account
originating the tx.  User provided metric prefix strings override the
Titan default and will appear in whatever Metrics reporter targets the
user configures, such as Ganglia or Graphite.

This commit is a probably-buggy refactoring baseline.  It reflects the
broad strokes structural changes that I wanted to do in Metrics
configuration, but I've probably messed up the details somewhere.
Additional bugfix commits likely to follow...
Fix Configuration loading exception

Replace Configuration#getLong(String) call with call to
Configuration#getLong(String, null) to avoid exception when the key is
unbound
Fix getSlice Metric name mangling
Fix broken getSlice.iterator Metric names
Make StandardTitanTx increment tx counters

Before this commit, StandardTitanTx created and configured its
counters properly, but never actually incremented them.  Added three
counter.inc() calls, one for begins, another for commits, another for
rollbacks.
Fix metrics.prefix config parsing

The logic attempting to read to read the metrics.prefix key was used
the wrong config subset; moved to
GraphDatabaseConfiguration#configureMetrics()
Change metrics prefix in non-transactional context

Some backend interactions happen outside any transaction, such as the
background ID buffer thread or the test cases.  These now have their
own Metrics name prefix.
Renamed GraphDBConf constants and updated javadocs

Refactoring the names of several Metrics-related config constants,
most of which I just introduced in commits over the past few days
Disable Metrics by default

Changed BASIC_METRICS_DEFAULT from true to false
Metrics-related Javadoc update