fix: shutdown grpc stubs properly when a subscriber is stopped

[hannahrogers-google]

Fixes #22

[codecov]

Codecov Report

Merging #74 into master will decrease coverage by <.01%.
The diff coverage is 82.6%.

@@             Coverage Diff              @@
##             master      #74      +/-   ##
============================================
- Coverage     78.94%   78.93%   -0.01%     
- Complexity      292      293       +1     
============================================
  Files            21       21              
  Lines          2588     2597       +9     
  Branches        129      129              
============================================
+ Hits           2043     2050       +7     
- Misses          490      492       +2     
  Partials         55       55

Impacted Files	Coverage Δ	Complexity Δ
.../com/google/cloud/pubsub/v1/MessageDispatcher.java	85.64% <100%> (ø)	25 <5> (ø)	⬇️
...c/main/java/com/google/cloud/pubsub/v1/Waiter.java	76.47% <100%> (ø)	6 <4> (?)
...in/java/com/google/cloud/pubsub/v1/Subscriber.java	79.67% <100%> (+0.11%)	22 <0> (ø)	⬇️
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	89.07% <100%> (ø)	43 <0> (ø)	⬇️
...cloud/pubsub/v1/StreamingSubscriberConnection.java	63.84% <50%> (-0.91%)	9 <0> (ø)
...oogle/cloud/pubsub/v1/stub/GrpcSubscriberStub.java	94.23% <0%> (+0.54%)	22% <0%> (+1%)	⬆️

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[hannahrogers-google]

This PR modifies the code to do the following:

1. Makes the MessageWaiter class more generic, so it can be used for all types of pending actions
2. Uses the Waiter class to track outstanding ack/modAck operations that have not completed
3. Waits for all outstanding ack operations to complete before shutting down a StreamingSubscriberConnection
4. Shuts down the GrpcSubscriberStub when a subscriber has been stopped.

This PR should fix issue #22 and issue #28. Using the steps to reproduce the issue (provided in #28), I was able to confirm the problem. With this fix, I confirmed that the issue is no longer present.