* Add round support for DS-HLL
Since the Cardinality aggregator has a "round" option to round off estimated
values generated from the HyperLogLog algorithm, add the same "round" option to
the DataSketches HLL Sketch module aggregators to be consistent.
* Fix checkstyle errors
* Change HllSketchSqlAggregator to do rounding
* Fix test for standard-compliant null handling mode
* Upgrade various build and doc links to https.
Where it wasn't possible to upgrade build-time dependencies to https,
I kept http in place but used hardcoded checksums or GPG keys to ensure
that artifacts fetched over http are verified properly.
* Switch to https://apache.org.