Fix ML types in CLI

Fix decoding of JSON values for ML model types
Fix intermittent test failure in TestRowOperators
Use StandardType constants in QueryResults
Remove extra space in LearnLibSvmClassifierAggregation
Save conf so that superclass methods have access to it via getConf()
Fix for row number window function optimization
Update approx_distinct to use more precise airlift HLL implementation
Allow specifying standard error for approx_distinct agggregation function
Add release notes for 0.88
[maven-release-plugin] prepare release 0.88
[maven-release-plugin] prepare for next development iteration
Upgrade to Airbase 30 and Airlift 0.99-SNAPSHOT
Convert CLI to java.util.Optional
Convert Kafka plugin to java.util.Optional
Convert verifier to java.util.Optional
Convert tests to java.util.Optional
Convert Hive plugin to java.util.Optional
Convert aggregations to java.util.Optional
Convert presto-main to use java.util.Optional