fix: update retry logic for operations that can be associated with a transaction

[vadimyushprakh]

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Fixes b/157596445 ☕️

Add a new exception handler for requests that may run within a transaction. The handler prevents retries on ABORTED error, since an aborted transaction cannot be re-used.

[codecov]

Codecov Report

Merging #144 into master will decrease coverage by 0.68%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #144      +/-   ##
============================================
- Coverage     58.98%   58.30%   -0.69%     
  Complexity      384      384              
============================================
  Files            46       47       +1     
  Lines          2304     2331      +27     
  Branches        254      261       +7     
============================================
  Hits           1359     1359              
- Misses          838      865      +27     
  Partials        107      107              

Impacted Files	Coverage Δ	Complexity Δ
...java/com/google/cloud/datastore/DatastoreImpl.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...atastore/TransactionOperationExceptionHandler.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)

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