Simplify core 2#13

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yilongli wants to merge 33 commits intoPlatformLab:masterfrom

yilongli:simplify-core-2

Commits on Oct 7, 2020

Commits on Oct 14, 2020

Commits on Oct 15, 2020

Commits on Oct 22, 2020

similar cleanups for Receiver
yilongli
committed

Commits on Oct 28, 2020

pulled in the changes that simplify Homa::InMessage + improvements based on Collin's comments
yilongli
committed

Commits on Nov 10, 2020

fixed some trivial issues raised in the code reviews
yilongli
committed

Commits on Nov 11, 2020

fixed the race conditions in Receiver due to reduced coverage of the bucket mutex
yilongli
committed

Commits on Nov 12, 2020

fixed race conditions on the Sender side
yilongli
committed