Make all thread pools create daemon threads

[elefeint]

Currently the thread pool for Pub/Sub's SubscriberStub keeps JVM from shutting down, causing non-web application to "hang".

This PR makes all Spring Cloud GCP threads daemon threads that do not prevent a Spring application from stopping.

This is a change in behavior for Pub/Sub-dependent command line applications that subscribe to a Pub/Sub topic with PubSubTemplate.subscribe(), and expect the application to run until externally terminated.

Fixes #2005.

[codecov]

Codecov Report

Merging #2010 into master will decrease coverage by 7.97%.
The diff coverage is 80%.

@@             Coverage Diff             @@
##             master   #2010      +/-   ##
===========================================
- Coverage     80.37%   72.4%   -7.98%     
+ Complexity     2115    1879     -236     
===========================================
  Files           243     243              
  Lines          6895    6900       +5     
  Branches        709     709              
===========================================
- Hits           5542    4996     -546     
- Misses         1058    1576     +518     
- Partials        295     328      +33

Flag	Coverage Δ	Complexity Δ
#integration	?	?
#unittests	72.4% <80%> (ø)	1879 <0> (ø)	⬇️

Impacted Files	Coverage Δ	Complexity Δ
...configure/vision/CloudVisionAutoConfiguration.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...toconfigure/pubsub/GcpPubSubAutoConfiguration.java	90.83% <100%> (+0.21%)	38 <0> (ø)	⬇️
...igure/trace/StackdriverTraceAutoConfiguration.java	69.49% <100%> (+0.52%)	10 <0> (ø)	⬇️
...a/spanner/repository/query/SpannerQueryMethod.java	0% <0%> (-100%)	0% <0%> (-6%)
...ository/config/FirestoreRepositoriesRegistrar.java	0% <0%> (-100%)	0% <0%> (-3%)
...sitory/support/FirestoreRepositoryFactoryBean.java	0% <0%> (-100%)	0% <0%> (-4%)
...pository/support/FirestoreQueryLookupStrategy.java	0% <0%> (-100%)	0% <0%> (-2%)
...figure/config/GcpConfigBootstrapConfiguration.java	0% <0%> (-100%)	0% <0%> (-2%)
...restore/repository/query/FirestoreQueryMethod.java	0% <0%> (-100%)	0% <0%> (-2%)
...ository/config/DatastoreRepositoriesRegistrar.java	0% <0%> (-100%)	0% <0%> (-3%)
... and 52 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8ce9877...3840092. Read the comment docs.

[meltsufin]

Thanks!

[meltsufin]

Wasn't this part of some other PR?

[elefeint]

... and this is why it's a best practice to delete branches after merging to master ...
// shame cube