* Merge Re-mount volumes on a restart or update #72

* Ensure the mutex is not copied, even when the nodeServer is copied by
  storing a pointer to the mutex, instead of the mutex itself

* Use Mutex instead of RWMutex, as having two readers of the variable at
  the same time means we are going to write the state at the same time,
  corrupting the state file on storage.

* Use private freestanding functions mutex as Mutex / RWMutex are not 
  recursive / re-entrant in Go.

* Improve the remounting logic by using a rate-limited amount of 
   background goroutines and adding timeouts values so that the service
   startup time is not dependent on the number of tracked volumes.

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Co-authored-by: Brian Kanya <[email protected]>