ACCUMULO-3136 Be a little more rigid on the ActiveScans we observe.

We might get scans by the system to the metadata table instead of
the scan we run in the test case. When this happens, the test will
timeout because the interrupt happens before the scan is running, and then
the scan just sits on the SlowIterator.
Merge branch '1.6.1-SNAPSHOT'

Conflicts:
	test/src/test/java/org/apache/accumulo/test/AllowScansToBeInterruptedIT.java