Allow the specification of a write window for retention policies

[davidby-influx]

Customer operating "shared" clusters frequently run into issues where a user will attempt to write data in to the cluster and the time is skewed either deeply into the future or significantly into the past.

Clean-up of this "bad data" is then problematic, time consuming, etc.

Add FutureWriteLimit and PastWriteLimit to retention policies. Points which are outside of now() + FutureWriteLimit or now() - PastWriteLimit will be rejected on write with a PartialWriteError.

The default will be zero for both values, which behaves as the current product.

There will be corresponding work for Enterprise and InfluxQL.