fs: encapsulate FSReqWrap more
In further preparation for the Promises enabled fs API, further encapsulate FSReqWrap details.
This is another bit of code extracted from the WIP fs promises PR: https://github.com/nodejs/node/pull/17739. Pulling this out in order to make the eventual fs promises PR easier to review.
/cc @addaleax @mcollina
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines