rfc : -Wshorten-64-to-32 warnings in folly liger dependencies

rfc : -Wshorten-64-to-32 warnings in folly liger dependencies

Summary:
This is not a fix but me seeking advice here. The context here is that in liger (our common network
stack for mobile platforms), we saw bugs due to truncation and want to enable these warnings. To find those in our code,
I had to first resolve the folly ones. I just got around by making the truncation explicit so that I can get to real errors.
Ideally I would like owners to examine each case and fix the original/accept the explicit truncate if its intended. Let me
know what you think. The last (least preferred) is for us to keep this as a liger only change. We have a couple of ways to
do that but it would be nice to fix.

N.B : this covers only liger dependencies

Test Plan: errors resolved after these changes.

Reviewed By: njormrod@fb.com

Subscribers: trunkagent, doug, shilin, njormrod, seanc, pgriess

FB internal diff: D1509355