Add mechanism to wait for items to be processed.

[martinmr]

Cache operations are handled asynchronously. Calling the Wait method will add an item to the
process queue and block until that item is processed. Useful to ensure all the previous items
have been processed before proceeding.

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This change is 

[manishrjain]

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @anurags92 and @karlmcguire)

[blixt]

Sorry for jumping into such an old PR but I was wondering if anyone has since tested the concurrency behavior of this approach? I would have expected there to be a SetSync(…) method instead which returns once the item has been set. By calling Set(…) followed by Wait(…) you may end up waiting for many other items being set by other threads, which on a busy server can average out to a large performance downgrade.