class CompletionQueueTag

FinalizeResult must be called before informing user code that the

operation bound to the underlying core completion queue tag has

completed. In practice, this means:

1. For the sync API - before returning from Pluck

2. For the CQ-based async API - before returning from Next

3. For the callback-based API - before invoking the user callback

This is the method that translates from core-side tag/status to

C++ API-observable tag/status.

The return value is the status of the operation (returning status is the

general behavior of this function). If this function returns false, the

tag is dropped and not returned from the completion queue: this concept is

for events that are observed at core but not requested by the user

application (e.g., server shutdown, for server unimplemented method

responses, or for cases where a server-side RPC doesn't have a completion

notification registered using AsyncNotifyWhenDone)