Remove synchronized on Tracer#getInstance

Move to the initialization on demand holder idiom to remove lock
contention after first initialization.
Make sure that no CME is possible in TraceTree.

HBase was seeing CME's sometimes in tests as the collections of spans
being created were being modified.  Create a copy so this isn't
possible.
Merge pull request #12 from elliottneilclark/cme_tracetree

Make sure that no CME is possible in TraceTree.
Merge pull request #11 from elliottneilclark/tracer_no_sync

Remove synchronized on Tracer#getInstance