fix: make awaitTermination and shutdown protected, since we already h…

[yirutang]

…ave close() method, it is confusing to have 3 shutdown methods

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3a5d013). Click here to learn what that means.
The diff coverage is 40.00%.

@@            Coverage Diff            @@
##             master     #330   +/-   ##
=========================================
  Coverage          ?   75.69%           
  Complexity        ?      465           
=========================================
  Files             ?       54           
  Lines             ?     3119           
  Branches          ?      175           
=========================================
  Hits              ?     2361           
  Misses            ?      652           
  Partials          ?      106           

Impacted Files	Coverage Δ	Complexity Δ
.../cloud/bigquery/storage/v1alpha2/StreamWriter.java	80.89% <40.00%> (ø)	33.00 <5.00> (?)
...age/v1/stub/readrows/ReadRowsRetryingCallable.java	95.23% <0.00%> (ø)	2.00% <0.00%> (?%)
...1beta2/stub/readrows/ReadRowsRetryingCallable.java	95.23% <0.00%> (ø)	2.00% <0.00%> (?%)
.../cloud/bigquery/storage/v1/BigQueryReadClient.java	52.94% <0.00%> (ø)	9.00% <0.00%> (?%)
...uery/storage/v1beta1/stub/BigQueryStorageStub.java	16.66% <0.00%> (ø)	1.00% <0.00%> (?%)
...storage/v1beta2/stub/BigQueryReadStubSettings.java	84.67% <0.00%> (ø)	12.00% <0.00%> (?%)
.../storage/v1beta1/stub/GrpcBigQueryStorageStub.java	93.12% <0.00%> (ø)	12.00% <0.00%> (?%)
...eta1/stub/EnhancedBigQueryStorageStubSettings.java	85.00% <0.00%> (ø)	10.00% <0.00%> (?%)
...1beta1/stub/readrows/ReadRowsRetryingCallable.java	95.23% <0.00%> (ø)	2.00% <0.00%> (?%)
...ud/bigquery/storage/v1/BaseBigQueryReadClient.java	43.90% <0.00%> (ø)	10.00% <0.00%> (?%)
... and 45 more

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