[FLINK-38810] Use splittable timers in Interval Join

[rkhachatryan]

What is the purpose of the change

On timer, TimeIntervalJoin potentially performs a lot of work iterating through all it's records in state.

And when multiple timers are triggered by a single watermark, checkpoint can be delayed significantly, effectively increasing e2e latency.

Recently, splittable timers were introduced to Flink for a limited set of operators in FLINK-20217.

TimeIntervalJoin can benefit from it as well.

[flinkbot]

CI report:

• e669940 Azure: FAILURE

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[pnowojski]

I think change LGTM, but doesn't it lack test coverage? Are there some an existing tests for delayed watermarks?