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
ACCUMULO-3358 Start the verify thread.
Merge branch '1.6'
ACCUMULO-3351 Remove use of deprecated trace code in 1.7.x
ACCUMULO-3355 Broke this IT in the process. Need to check a few times to give the tracer time to write to the table.
Merge branch '1.6'
ACCUMULO-3299 Ensure tests run properly scoped

  Ensure some tests which were not running, because of their name are running
  during the build. Made some MiniAccumuloCluster-based tests ITs, and attempted
  to prevent tests from writing to /tmp (use target/ instead).
ACCUMULO-3362 Fix incorrect javadoc for StatsCombiner
Merge branch '1.5' into 1.6
Merge branch '1.6'
ACCUMULO-3199 Internal refactor to add ClientContext

  This patch introduces a new ClientContext object that contains Credentials,
  Instance, and Configuration provided from the client API. This new object is
  passed around internally in place of the previous three objects. An
  AccumuloServerContext is also introduced, which extends the ClientContext.
  Together, these objects ensure the proper configuration, credentials, and
  everything else needed to communicate with other system components are
  available to any RPC-related code.

  These new object types also reduce the need to create multiple references to
  commonly used internal objects (such as HdfsZooInstance and
  SystemCredentials), and avoids storing information in static fields. As a
  side-effect, this should allow for better testing with mocked components.

  This fixes ACCUMULO-3252, and may lay some groundwork for ACCUMULO-2589.