aggressive retention purge of MERGING backups

[gsmolk]

Example:

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FAIL: test_failed_merge_redundancy_retention (tests.retention.RetentionTest)
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Traceback (most recent call last):
  File "tests/retention.py", line 1588, in test_failed_merge_redundancy_retention
    self.show_pb(backup_dir, 'node', full_id)['status'],
  File "tests/helpers/ptrack_helpers.py", line 886, in show_pb
    self.assertTrue(False, "Failed to find backup with ID: {0}".format(backup_id))
AssertionError: Failed to find backup with ID: PWYII3

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Ran 1 test in 36.689s

FAILED (failures=1)

Reported by Alexander Nikitin