fdb-record-layer Resolves #1424: Add count and countNonNull to streaming aggregate

Add support and tests for count and countNonNull: #1424. Specifically, the following are true for aggregations and counts:

Zero records aggregate to NULL (e.g. Sum() and Avg() without any records return null).
Exception to the rule above is Count, where: 2.1. count() with no aggregation criteria and no records is 0. 2.2. count() with some aggregation criteria and no records returns empty stream. (Same as SQL).
Empty table (no records at all ) returns empty result (no records).
All aggregations ignore null values: They are considered a no-op for avg(), sum() etc.
Exception to the above rule is Count where count() with will include null values.
In order to exclude null records from the count, use the countNonNull() aggregation.

Oct 11 '21 18:10 ohadzeliger

Oct 11 '21 18:10 foundationdb-ci

Oct 11 '21 18:10 foundationdb-ci

Oct 11 '21 19:10 foundationdb-ci

CodeBuild project: fdb-record-layer-pull-request-build-proto2
Commit ID: 9b51f5a6c18417deb7a41b0e132d51e33344fa09
Result: FAILED
Error: Error while executing command: ./gradlew --console=plain -b ./build.gradle build destructiveTest sonarqube -PcoreNotStrict -PreleaseBuild=false -PpublishBuild=false -PspotbugsEnableHtmlReport. Reason: exit status 1
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Oct 11 '21 20:10 foundationdb-ci