Make `fold` work on changefeeds.
Make `fold` work on changefeeds.

Fixes #5800.

robertjpayne/rethinkdb,mbroadst/rethinkdb,JackieXie168/rethinkdb,bchavez/rethinkdb,eliangidoni/rethinkdb,bpradipt/rethinkdb,bchavez/rethinkdb,JackieXie168/rethinkdb,eliangidoni/rethinkdb,grandquista/rethinkdb,bpradipt/rethinkdb,robertjpayne/rethinkdb,robertjpayne/rethinkdb,jmptrader/rethinkdb,JackieXie168/rethinkdb,robertjpayne/rethinkdb,eliangidoni/rethinkdb,mbroadst/rethinkdb,catroot/rethinkdb,JackieXie168/rethinkdb,robertjpayne/rethinkdb,jmptrader/rethinkdb,mbroadst/rethinkdb,bchavez/rethinkdb,bchavez/rethinkdb,bpradipt/rethinkdb,mbroadst/rethinkdb,JackieXie168/rethinkdb,bpradipt/rethinkdb,catroot/rethinkdb,eliangidoni/rethinkdb,jmptrader/rethinkdb,catroot/rethinkdb,jmptrader/rethinkdb,bpradipt/rethinkdb,grandquista/rethinkdb,catroot/rethinkdb,eliangidoni/rethinkdb,bchavez/rethinkdb,eliangidoni/rethinkdb,bpradipt/rethinkdb,bpradipt/rethinkdb,eliangidoni/rethinkdb,jmptrader/rethinkdb,bpradipt/rethinkdb,JackieXie168/rethinkdb,JackieXie168/rethinkdb,JackieXie168/rethinkdb,mbroadst/rethinkdb,bpradipt/rethinkdb,grandquista/rethinkdb,mbroadst/rethinkdb,robertjpayne/rethinkdb,mbroadst/rethinkdb,mbroadst/rethinkdb,bchavez/rethinkdb,jmptrader/rethinkdb,bchavez/rethinkdb,catroot/rethinkdb,jmptrader/rethinkdb,bchavez/rethinkdb,catroot/rethinkdb,JackieXie168/rethinkdb,robertjpayne/rethinkdb,grandquista/rethinkdb,grandquista/rethinkdb,mbroadst/rethinkdb,robertjpayne/rethinkdb,eliangidoni/rethinkdb,eliangidoni/rethinkdb,grandquista/rethinkdb,bchavez/rethinkdb,grandquista/rethinkdb,grandquista/rethinkdb,robertjpayne/rethinkdb,jmptrader/rethinkdb,catroot/rethinkdb,grandquista/rethinkdb,catroot/rethinkdb