test for multiple dead compactions

[ArbaazKhan1]

Closes issue #5084

Created testCleanUpMultipleDeadCompactions() starts with creating 3 compaction jobs and removes 2 from the metaData "simulating" its a dead compaction and then check coordinator.getRuning() to see if dead compactions still exist or not.

[DomGarguilo]

As far as I can tell this test case looks like it covers the logic added in #5083. @keith-turner I requested your review as you were the one who created the mentioned PR. Maybe you could take a quick look as well.

[keith-turner]

This unit test is not testing the metadata cleanup related to dead compactions. Would be better to do this in an IT, would be difficult to test the metadata updates in a unit test. Could do something like the following in an IT.

1. Start 6 ExternalDoNothingCompactor processes
2. Create a table w/ 3 tablets
3. Limit compactions to 10 files per compaction
4. Configure table to use the 6 ExternalDoNothingCompactor
5. write 30 files to first tablet, want this to start 3 compactions for the tablet
6. write 20 files to 2nd tablet, want this to start 2 compactions for the tablet
7. write 10 files to 3rd tablet, want this to start 1 compaction for the tablet
8. Continually read metadata table looking for the 6 compactions, when found remember their unique ids
9. Kill the 6 ExternalDoNothingCompactor
10. Continually read metadata table waiting for the 6 compaction ids to disappear (the compaction coordinator should detect the processes died and clean up any per tablet compaction entries).
11. Read the running set from the coordinator until the 6 ids are gone

This test would cover what this test is attempting to cover and more.