fix: SessionNotFound was not retried for AsyncTransactionManager

[olavloite]

SessionNotFoundException was not retried if these happened when using AsyncTransactionManager. These exceptions are now handled explicitly in the following way:

1. The underlying session that is used for the transaction is replaced with a new session from the pool.
2. The SessionNotFoundException is translated to an AbortedException, which again will be retried, assuming that the client application has implemented a normal retry loop, as is required when using an AsyncTransactionManager.

[thiagotnunes]

LGTM

[codecov]

Codecov Report

Merging #552 into master will increase coverage by 0.13%.
The diff coverage is 89.10%.

@@             Coverage Diff              @@
##             master     #552      +/-   ##
============================================
+ Coverage     83.93%   84.06%   +0.13%     
- Complexity     2501     2504       +3     
============================================
  Files           141      141              
  Lines         13766    13806      +40     
  Branches       1315     1317       +2     
============================================
+ Hits          11554    11606      +52     
+ Misses         1667     1654      -13     
- Partials        545      546       +1     

Impacted Files	Coverage Δ	Complexity Δ
...gle/cloud/spanner/AsyncTransactionManagerImpl.java	70.42% <33.33%> (-1.01%)	12.00 <1.00> (ø)
...ain/java/com/google/cloud/spanner/SessionPool.java	88.33% <88.46%> (+1.43%)	71.00 <0.00> (ø)
...ud/spanner/SessionPoolAsyncTransactionManager.java	86.55% <100.00%> (+1.64%)	11.00 <1.00> (+2.00)
.../google/cloud/spanner/SpannerExceptionFactory.java	82.92% <0.00%> (+2.43%)	40.00% <0.00%> (+1.00%)

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