Cherry-pick pull #3529 into Release 2.1.0

[tgroh]

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BEAM-2575 ApexRunner doesn't emit watermarks for additional outputs

[tgroh]

R: @jbonofre
CC: @tweise

[jbonofre]

LGTM, I gonna merge. Thanks !

[jbonofre]

Waiting the end of Jenkins before merging.

[coveralls]

Changes Unknown when pulling 96ff220 on tgroh:cherrypick_3529 into ** on apache:release-2.1.0**.

[tgroh]

Jenkins is green. Merge at will.