+act Add japi.Pair and japi.Predicate for streams

(cherry picked from commit 42ce37bb57884c7051d46a1e80b4a4ba4187adf4)

=act Add @SerialVersionUID to japi.Pair

(cherry picked from commit adb2c9bdc20effc84132daaf37e563fb430bf230)
Merge pull request #15199 from akka/wip-stream-japi-master-patriknw

+act Add japi.Pair and japi.Predicate for streams (for validation)