ACCUMULO-2450 Backport of ACCUMULO-2438 to 1.4: ensure that we watch separate sets of tables for merges
(cherry picked from commit 12b53b043b60368fb647db9c0a15916171d4504b)

Original authors: Eric Newton (1.6+), Josh Elser (1.5 backport)

This backport does not include the new DeleteRowsTest, as no location for it is obvious
under the 1.4 development branch.

Conflicts:
	src/server/src/main/java/org/apache/accumulo/server/master/Master.java