fix: query could hang transaction if ResultSet#next() is not called

[olavloite]

If the first statement of a read/write transaction was a query or a read operation, and the application would not call ResultSet#next() on the return result, the transaction would hang indefinitely as the query would be marked as the one that should initiate the transaction (inline the BeginTransaction option). The query would however never be executed, as the actual query execution is deferred until the first call to ResultSet#next().

Fixes #641

[codecov]

Codecov Report

Merging #643 (39a3a85) into master (79fad94) will increase coverage by 0.01%.
The diff coverage is 94.73%.

@@             Coverage Diff              @@
##             master     #643      +/-   ##
============================================
+ Coverage     84.04%   84.06%   +0.01%     
+ Complexity     2503     2498       -5     
============================================
  Files           141      141              
  Lines         13810    13812       +2     
  Branches       1318     1317       -1     
============================================
+ Hits          11607    11611       +4     
+ Misses         1657     1655       -2     
  Partials        546      546              

Impacted Files	Coverage Δ	Complexity Δ
...va/com/google/cloud/spanner/AbstractResultSet.java	83.48% <85.71%> (+0.09%)	28.00 <0.00> (ø)
.../com/google/cloud/spanner/AbstractReadContext.java	86.15% <100.00%> (-0.05%)	47.00 <0.00> (-6.00)
...om/google/cloud/spanner/TransactionRunnerImpl.java	84.27% <100.00%> (ø)	9.00 <0.00> (ø)
.../google/cloud/spanner/SpannerExceptionFactory.java	82.92% <0.00%> (+2.43%)	40.00% <0.00%> (+1.00%)

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