Kraken: Change Mutex in FileStore to RWMutex
Kraken: Change Mutex in FileStore to RWMutex

Summary: so most operations will be non-blocking. should have done this earlier.

Reviewers: #kraken, xiaojian, igor

Reviewed By: #kraken, igor

Subscribers: jenkins

Differential Revision: https://code.uberinternal.com/D871213

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