[BTS-664] Hotbackup transfer job cleanup part 2

[neunhoef]

Even more cleanups for hotbackup transfer jobs.

It was found that the previous round of cleanups did not consider
old, unfinished, seemingly ongoing jobs and transfer locks.

This PR adds cleanup for all those to keep the agency size under
control.

This PR is accompanied by an enterprise repo PR: https://github.com/arangodb/enterprise/pull/826

Scope & Purpose

(Please describe the changes in this PR for reviewers - mandatory)

• [*] 💩 Bugfix (requires CHANGELOG entry)
• [*] 📖 CHANGELOG entry made

Backports:

• [*] This is the forward port of a fix for 3.8. See Hotbackup transfer job cleanup part 2 BTS-664 #15256

Related Information

(Please reference tickets / specification / other PRs etc)

• [*] GitHub issue / Jira ticket number: https://arangodb.atlassian.net/browse/BTS-664

• [*] See also

  Hotbackup transfer job cleanup part 2 https://github.com/arangodb/enterprise/pull/826

Testing & Verification

(Please pick either of the following options)

• [*] The behavior in this PR was manually tested
• [*] This change is already covered by existing tests, such as (please describe tests).
• [] This PR adds tests that were used to verify all changes:
  - [] Added new C++ Unit tests

[kvahed]

LGTM