Add redis-based quota.Manager

[adunham-stripe]

What

This pull request adds two new packages:

1. A Redis-based token bucket algorithm (under quota/redis/redistb). The underlying Lua script is based on what Stripe uses for our internal rate-limiters, is atomic, can handles tens of thousands of requests per second with no trouble, and is easily scaled via Redis Cluster.
2. A Redis-based quota.Manager implementation that uses it (under quota/redis/redisqm). This package is currently fairly basic; all configuration is performed via code, not stored in Redis or another datastore. Note that it doesn't acquire multiple quota.Specs transactionally, but rather in order. This should be easy to extend in the future, if necessary.

Checklist

• I have updated the CHANGELOG.
  • Adjust the draft version number according to semantic versioning rules.
• I have updated documentation accordingly (including the feature implementation matrix).

[AlCutter]

Thanks Andrew, this is super cool!
A few minor nits inline.

[AlCutter]

Ah, there are a couple of other bits the presubmit picked up, too:

License check:

./quota/redis/redistb/embed_redis.go:10:license header not found
./quota/redis/redistb/gen.go:10:license header not found
./quota/redis/redistb/redistb_test.go:10:license header not found
./quota/redis/redistb/redistb.go:10:license header not found

Travis integration:
Looks like we need to install and spin up a local redis daemon, maybe?

--- FAIL: TestRedisTokenBucketIntegration (0.00s)
    redistb_test.go:65: failed to load script: dial tcp 127.0.0.1:6379: connect: connection refused

[AlCutter]

Travis integration:
Looks like we need to install and spin up a local redis daemon, maybe?

--- FAIL: TestRedisTokenBucketIntegration (0.00s)
    redistb_test.go:65: failed to load script: dial tcp 127.0.0.1:6379: connect: connection refused

Seems like it's easy enough to get a redis server by editing the services section in our .travis.yml file: https://docs.travis-ci.com/user/database-setup/#redis

[codecov]

Codecov Report

Merging #1977 into master will increase coverage by 0.08%.
The diff coverage is 72.22%.

@@            Coverage Diff             @@
##           master    #1977      +/-   ##
==========================================
+ Coverage   58.15%   58.23%   +0.08%     
==========================================
  Files         114      115       +1     
  Lines        9678     9750      +72     
==========================================
+ Hits         5628     5678      +50     
- Misses       3553     3572      +19     
- Partials      497      500       +3

Impacted Files	Coverage Δ
quota/redis/redistb/redistb.go	72.22% <72.22%> (ø)
log/operation_manager.go	87.9% <0%> (-0.94%)	⬇️

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[adunham-stripe]

@AlCutter - Thanks for the feedback! Pushed changes, and it looks like tests are all passing now, including those using the Redis server 🎉

[AlCutter]

@adunham-stripe Many thanks! Looking forward to seeing what else you might come up with :)

[adunham-stripe]

@AlCutter - And thank you for the quick review!

[RJPercival]

nit: You're missing a package comment here.