fix: allow DefaulBackgroundThreads to self-destroy

[dopiera]

The last reference to AutomaticallyCreatedBackgroundThreads may be in
the CompletionQueue owned by itself. This means that
~AutomaticallyCreatedBackgroundThreads() will be executed in the
thread belonging to it. Before this PR this resulted in a deadlock. This
PR fixes it.

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This change is 

[coryan]

When we have ran into this problem before we solved it by separating the ownership of the thread pool from the users of the CQ. For example, in pubsub we do this:

google-cloud-cpp/google/cloud/pubsub/publisher_connection.cc

Line 35 in 36bfcef

class ContainingPublisherConnection : public PublisherConnection {

In spanner we did this:

googleapis/google-cloud-cpp-spanner#1397

[codecov]

Codecov Report

Merging #5681 (088f5a3) into master (8275f58) will decrease coverage by 0.02%.
The diff coverage is 96.50%.

@@            Coverage Diff             @@
##           master    #5681      +/-   ##
==========================================
- Coverage   95.44%   95.42%   -0.03%     
==========================================
  Files        1106     1106              
  Lines      100378   100727     +349     
==========================================
+ Hits        95807    96116     +309     
- Misses       4571     4611      +40     

Impacted Files	Coverage Δ
google/cloud/internal/background_threads_impl.h	100.00% <ø> (ø)
google/cloud/internal/background_threads_impl.cc	92.59% <80.00%> (-7.41%)	⬇️
google/cloud/bigtable/internal/common_client.cc	93.75% <94.44%> (+2.57%)	⬆️
google/cloud/bigtable/internal/common_client.h	96.82% <100.00%> (+0.15%)	⬆️
...ogle/cloud/bigtable/internal/common_client_test.cc	100.00% <100.00%> (ø)
...le/cloud/internal/default_completion_queue_impl.cc	96.98% <100.00%> (+0.21%)	⬆️
google/cloud/internal/async_connection_ready.h	50.00% <0.00%> (-50.00%)	⬇️
...e/cloud/storage/tests/key_file_integration_test.cc	91.52% <0.00%> (-2.71%)	⬇️
...oud/storage/tests/curl_request_integration_test.cc	96.57% <0.00%> (-2.50%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	86.59% <0.00%> (-2.07%)	⬇️
... and 98 more

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[coryan]

Closing for now, we can reopen later.