[refactor] use match%lwt instead of weird >|= infix
[refactor] use match%lwt instead of weird >|= infix

Reviewed By: panagosg7

Differential Revision: D24296816

fbshipit-source-id: adc471aff1e737d1de36b5bdf24a543acb00654e

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow