docstore/memdocstore: apply limit after sorting

[jba]

Fixes #2977

[jba]

I don't understand the CI failure (https://travis-ci.com/github/google/go-cloud/jobs/491509068), but it doesn't seem connected to this PR.

[vangent]

Agree, it looks like a flake (Zookeeper failed to start?). You might have to re-push to trigger it to try again; the "Retry" button in Travis just gives me an "Oh No" error.

[codecov]

Codecov Report

Merging #2978 (59d2e60) into master (9529703) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2978      +/-   ##
==========================================
+ Coverage   69.58%   69.65%   +0.06%     
==========================================
  Files         111      111              
  Lines       11556    11556              
==========================================
+ Hits         8041     8049       +8     
+ Misses       2853     2842      -11     
- Partials      662      665       +3     

Impacted Files	Coverage Δ
docstore/memdocstore/query.go	78.94% <100.00%> (ø)
docstore/docstore.go	87.22% <0.00%> (-2.21%)	⬇️
blob/s3blob/s3blob.go	89.47% <0.00%> (+0.52%)	⬆️
runtimevar/awssecretsmanager/awssecretsmanager.go	74.19% <0.00%> (+2.41%)	⬆️
server/health/sqlhealth/sqlhealth.go	78.37% <0.00%> (+21.62%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 32ad8e5...59d2e60. Read the comment docs.