Add configurable shard duration to retention policies

[oldmantaiter]

• CHANGELOG.md updated
• Rebased/mergable
• Tests pass
• Sign CLA (if not already signed)

Allows configuration of shard group duration at database creation, and retention
policy create/alter time.

Query examples:

CREATE DATABASE testdb WITH DURATION 90d REPLICATION 1 SHARD DURATION 30m NAME rp_testdb 
CREATE RETENTION POLICY rp_testdb2 ON testdb DURATION INF REPLICATION 1 SHARD DURATION 30m
ALTER RETENTION POLICY rp_testdb2 ON testdb SHARD DURATION 1h

This can be useful with long duration retention policies with lots of data, where
you can split into smaller shards to relieve memory pressure.

[oldmantaiter]

Sorry for all the rebases - realized after about 5 failed circle ci checks that I should be testing locally. The one thing I have not done is regenerate the pb files. Let me know if this looks ok and I'll get that part done as well.

[jwilder]

cc @pauldix

[oldmantaiter]

Realized my commit message had the wrong syntax, fixed that and updated the PR comment

[jwilder]

@oldmantaiter This looks great. Would you be able to modify the syntax to be SHARD DURATION?

Other than that, I think it just needs a rebase.

[oldmantaiter]

@jwilder No problem, I'll take a look. As for any protobuf changes, are those considered part of PRs since the single node change in master?

[oldmantaiter]

@jwilder Updated to SHARD DURATION

[jwilder]

@dgnorton Can you take a look?

[dgnorton]

+1 lgtm

[jwilder]

👍 @oldmantaiter Looks like it needs another rebase.

[oldmantaiter]

@jwilder rebased

[jwilder]

Thanks @oldmantaiter!