feat: async work as functional interface

[thiagotnunes]

Marks the async work interface as a functional interface.

This PR also modifies all of its usages to the lambda syntax. Only syntatic changes were made, no logic changes were made here.

[codecov]

Codecov Report

Merging #1068 (a8cf021) into master (1d4eed4) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1068      +/-   ##
============================================
+ Coverage     85.24%   85.26%   +0.02%     
- Complexity     2658     2661       +3     
============================================
  Files           155      155              
  Lines         14453    14453              
  Branches       1362     1362              
============================================
+ Hits          12320    12324       +4     
+ Misses         1565     1560       -5     
- Partials        568      569       +1     

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	18.00% <0.00%> (+1.00%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.12% <0.00%> (+0.19%)	73.00% <0.00%> (+1.00%)
.../google/cloud/spanner/SpannerExceptionFactory.java	89.81% <0.00%> (+1.85%)	49.00% <0.00%> (+1.00%)

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