Bot should check whether PRs are already open with update

@bsipocz commented on [Fri Mar 02 2018](https://github.com/regro/cf-graph/issues/51)

First of all, thank you for this bot, I'm sure many maintainers will agree that this is a great to have feature to conda-forge.

I have one feature request though. We usually include the conda-forge update in our release procedure, so it happened already a few times that there is already a version update PR (usually waiting for CIs to pass) when the bot opens one, too clogging the CI services even more. I think it would be rather awesome if it would check not only the main repo, but the content of already opened PRs, too.

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@CJ-Wright commented on [Fri Mar 02 2018](https://github.com/regro/cf-graph/issues/51#issuecomment-370067673)

Thank you for reporting!

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@CJ-Wright commented on [Fri Mar 02 2018](https://github.com/regro/cf-graph/issues/51#issuecomment-370073409)

This may need its own script and/or worker since it may be GH API heavy.
My understanding of what needs to happen:
```python
feedstocks = get_feedstocks()
for feedstock in feedstocks:
for PR in feedstock.PRs():
get_yaml()
update_node_attributes_with_new_info()
```

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@isuruf commented on [Fri Mar 02 2018](https://github.com/regro/cf-graph/issues/51#issuecomment-370074355)

Or the linter can write to a file/db on each PR and the bot could check this file/db

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@CJ-Wright commented on [Fri Mar 02 2018](https://github.com/regro/cf-graph/issues/51#issuecomment-370076988)

This may need atomic-like operations on the graph.
I'll open another issue to discuss that.
See: https://github.com/regro/cf-graph/issues/52

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@jakirkham commented on [Fri Mar 02 2018](https://github.com/regro/cf-graph/issues/51#issuecomment-370085307)

Definitely agree with this issue. Though I wonder to what extent this is a consequence of the bot recently coming online vs. a recurrent problem we will face well into the future (if not otherwise addressed).

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@bsipocz commented on [Fri Mar 02 2018](https://github.com/regro/cf-graph/issues/51#issuecomment-370087333)

@jakirkham - you're probably right, if this bot becomes the default behaviour I suspect most maintainers will top opening those update PRs the first place. However in that case having a way to opt out may be useful.

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@CJ-Wright commented on [Fri Mar 02 2018](https://github.com/regro/cf-graph/issues/51#issuecomment-370089568)

@bsipocz although the bot is currently pushing the CI's rather hard, I think it will get easier once we enter steady state (and finish running through all the packages). At that point I think it would be ok to just close the bot's PRs. My assumption (which may not be true) is that the rate of version bumps will be slow enough that the bot opening an erroneous PR would not be too burdensome.