Pagination for list bucket

[shabbyrobe]

This implements pagination for both versions of the "list bucket" API:

• https://docs.aws.amazon.com/AmazonS3/latest/API/v2-RESTBucketGET.html
• https://docs.aws.amazon.com/AmazonS3/latest/API/RESTBucketGET.html

I'm using a bit of a trick with the ContinuationToken to save backend implementers from having to implement this twice; all the S3-mandated hacks are pushed into GoFakeS3.

Only the memory backend implements the pagination for now; I'm still trying to think of a good way to do it in the bolt backend. I might try and do that as a separate PR.

NOTE - this PR is based on #28 and depends on that merge, so it should stay WIP until that one has made it through review into master.

TODO:

• Tests

[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by 1.32%.
The diff coverage is 78.61%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage      65%   66.32%   +1.32%     
==========================================
  Files          26       27       +1     
  Lines        2260     2355      +95     
==========================================
+ Hits         1469     1562      +93     
  Misses        568      568              
- Partials      223      225       +2

Impacted Files	Coverage Δ
messages.go	70.75% <ø> (-3.2%)	⬇️
error.go	66.66% <0%> (-1.63%)	⬇️
util.go	68.96% <0%> (ø)	⬆️
routing.go	83.33% <100%> (+0.6%)	⬆️
option.go	70% <100%> (+3.33%)	⬆️
backend.go	100% <100%> (ø)
backend/s3mem/bucket.go	79.56% <100%> (+0.45%)	⬆️
range.go	83.33% <100%> (-0.54%)	⬇️
uploader.go	85.31% <100%> (-0.09%)	⬇️
prefix.go	96.96% <100%> (+6.34%)	⬆️
... and 9 more

---

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[shabbyrobe]

Alright I think I'm done tinkering here; I reckon this one's good to go! (pending any feedback, of course!)

[johannesboyne]

Man, I really like that! Implements a super important part of the S3 interface!

[shabbyrobe]

Thanks heaps for merging!

I've done a bit more digging and found that there are a lot of fussy edge cases that need to be handled before it'll be possible to work out a convenient way to roll this into the bolt and afero backends. I think the next time I get a block of time to look at stuff I might start cleaning up all the messy tests I've written recently and start looking at the backend tester; that'll make it a lot easier to start dealing with the finnicky bits.