rename "slavedest" to "workerdest" in buildbot-worker's downloadFile command

[rutsky]

This is expected backward incompatible master/worker API change.

Based on PR #2239.

Release notes are in WIP branch: https://github.com/rutsky/buildbot/blob/simple-e2e-test/master/docs/index.rst
I'll cherry-pick them when dependent PRs will be merged.

[codecov-io]

Current coverage is 84.28%

Merging #2244 into master will increase coverage by <.01%

1. 7 files (not in diff) in ...ldbot_worker/scripts were created. more
2. 6 files (not in diff) in ...dbot_worker/commands were created. more
3. 11 files (not in diff) in ...rker/buildbot_worker were created. more
4. 2 files (not in diff) in ...ster/buildbot/worker were modified. more
   • Hits +3
5. 2 files (not in diff) in master/buildbot/db were modified. more
   • Misses +9
   • Hits +16
6. 1 files (not in diff) in master/buildbot were modified. more
   • Hits +5

@@             master      #2244   diff @@
==========================================
  Files           359        359          
  Lines         34410      34422    +12   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          29000      29012    +12   
  Misses         5410       5410          
  Partials          0          0          

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[rutsky]

Looks like I missed some changes in tests — need to update tests.

[rutsky]

Tests coverage should be fixed now.

[rutsky]

Rebased, temporary workarounds removed. Still based on PR #2239, but it's ok to merge both.

[tardyp]

needs rebase

[rutsky]

Rebased