fix: properly shutdown subscriber stub on permanent streaming pull failure

[hannahrogers-google]

When a subscriber fails permanently, we are not shutting down the stub, like we do during a normal shutdown. Making a change to shut down the subscriber stub on failure. Before shutting down the subscriber stub, we must first shut down the MessageDispatcher to verifying that all acks are processed first.

Fixes #315 ☕️

[codecov]

Codecov Report

Merging #539 (f80de80) into master (d8728e9) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #539      +/-   ##
============================================
+ Coverage     78.79%   78.81%   +0.02%     
- Complexity      376      378       +2     
============================================
  Files            27       27              
  Lines          3414     3418       +4     
  Branches        171      171              
============================================
+ Hits           2690     2694       +4     
  Misses          651      651              
  Partials         73       73              

Impacted Files	Coverage Δ	Complexity Δ
...cloud/pubsub/v1/StreamingSubscriberConnection.java	67.30% <100.00%> (+0.64%)	15.00 <2.00> (+1.00)
...in/java/com/google/cloud/pubsub/v1/Subscriber.java	80.61% <100.00%> (+0.09%)	23.00 <1.00> (+1.00)

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