ACCUMULO-3351 Try to reset the writer if we don't have one but the table exists.

We have to avoid the case where we spin indefinitely trying to process
traces (but cannot because we don't have a writer). If we don't have
a writer, but the trace table exists, try to get a new writer.
ACCUMULO-3355 Catch the case when we find no traces and quit early.

If we don't quit early, we end up creating this absurd date and sending
it back to the user. Also fixed two tests that were unnecessarily
nuking the entire trace table. The Scanner they create is bounded for
their trace session; there is no reason to delete the rest of the records.
Merge branch '1.6'

Conflicts:
	server/tracer/src/main/java/org/apache/accumulo/tracer/TraceDump.java