Only cache the latest items in the time window
OperatorBuffer
OperatorConcat
Rename concat test
Merge pull request #1065 from zsxwing/skip-last-timed

Optimize OperatorSkipLastTimed
Merge pull request #1073 from akarnokd/OperatorBuffer

OperatorBuffer
Merge pull request #1074 from akarnokd/OperatorConcat

OperatorConcat