Adds a non-blocking close method to Connection to support APIs that should offer only non-blocking methods to consumers (e.g. R2DBC).

The Connection interface is marked with InternalApi and documented with Internal connection API for Google Cloud Spanner. This interface may introduce breaking changes without prior notice.

cc @elefeint

Codecov Report

Merging #984 (dcbf73b) into master (7af1951) will decrease coverage by 0.01%.
The diff coverage is 95.23%.

@@             Coverage Diff              @@
##             master     #984      +/-   ##
============================================
- Coverage     85.11%   85.09%   -0.02%     
+ Complexity     2618     2617       -1     
============================================
  Files           154      154              
  Lines         14310    14319       +9     
  Branches       1331     1331              
============================================
+ Hits          12180    12185       +5     
- Misses         1564     1570       +6     
+ Partials        566      564       -2     

Impacted Files	Coverage Δ	Complexity Δ
...om/google/cloud/spanner/connection/Connection.java	100.00% <ø> (ø)	0.00 <0.00> (ø)
...oogle/cloud/spanner/connection/ConnectionImpl.java	84.13% <95.23%> (+0.08%)	181.00 <1.00> (+1.00)
...le/cloud/spanner/connection/StatementExecutor.java	76.92% <0.00%> (-3.85%)	8.00% <0.00%> (-1.00%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.12% <0.00%> (-0.20%)	72.00% <0.00%> (-1.00%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	17.00% <0.00%> (-1.00%)
.../google/cloud/spanner/AbstractLazyInitializer.java	100.00% <0.00%> (+7.14%)	5.00% <0.00%> (+1.00%)

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