Adds a number of safeguards against hanging transactions if the first statement of a transaction for whatever reason fails to return a transaction ID, or otherwise returns an unexpected error. This PR contains the following safeguards:

1. If a statement requested a new transaction from Cloud Spanner and receives a response without a transaction ID, the statement will throw an exception. This should theoretically never happen. If it did happen, it would cause the second statement in the transaction to be 'stuck', as it would wait indefinitely for the first statement to return a transaction.
2. If the first statement of a transaction throws an unexpected error (a Throwable that is not a SpannerException), the exception will be translated to a SpannerException and handled as such. This error will mark the transaction as unusable, as the first statement failed to return a transaction.
3. If the first statement fails to respond at all (no response, no error), the second statement will wait at most 60 seconds for a transaction ID to be returned, and then fail with a ABORTED error. This will cause the transaction to be retried automatically.

Fixes #799

Codecov Report

Merging #800 (02a886b) into master (1a71e50) will increase coverage by 0.08%.
The diff coverage is 92.85%.

@@             Coverage Diff              @@
##             master     #800      +/-   ##
============================================
+ Coverage     85.01%   85.09%   +0.08%     
- Complexity     2562     2564       +2     
============================================
  Files           143      143              
  Lines         14015    14030      +15     
  Branches       1341     1341              
============================================
+ Hits          11915    11939      +24     
+ Misses         1537     1531       -6     
+ Partials        563      560       -3     

Impacted Files	Coverage Δ	Complexity Δ
...om/google/cloud/spanner/TransactionRunnerImpl.java	85.74% <92.10%> (+0.87%)	9.00 <0.00> (ø)
.../com/google/cloud/spanner/AbstractReadContext.java	87.07% <100.00%> (ø)	48.00 <1.00> (ø)
...va/com/google/cloud/spanner/AbstractResultSet.java	83.30% <100.00%> (+0.03%)	28.00 <0.00> (ø)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.02% <0.00%> (+0.19%)	71.00% <0.00%> (ø%)
...ud/spanner/SessionPoolAsyncTransactionManager.java	87.30% <0.00%> (+1.58%)	13.00% <0.00%> (+2.00%)
...m/google/cloud/spanner/connection/SpannerPool.java	86.11% <0.00%> (+1.66%)	31.00% <0.00%> (ø%)

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Thanks for this fixes @olavloite, really appreciate it.