Manager migration set cleanup does not consider the root table.

[keith-turner]

The following code exists to cleanup the manager migration set for the case of tablets splitting. It does not seem to consider the metadata table when cleaning up. The metadata table can split. Seems like it could also scan the root tablet.

accumulo/server/manager/src/main/java/org/apache/accumulo/manager/Manager.java

Line 709 in 7c6572c

private void cleanupNonexistentMigrations(final AccumuloClient accumuloClient)

[keith-turner]

#4474 is modifying the same code, so would be good to merge it before working on this