Fix transaction configuration for single-use transactions

[jdpedrie]

Database::execute() runs operations within a single-use transaction, but due to the order in which the transaction was configured, single-use configuration options (maxStaleness and minReadTimestamp) were not available to that method.

Fixes #1830.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@e66cb6d). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1844   +/-   ##
=========================================
  Coverage          ?   91.58%           
  Complexity        ?     4408           
=========================================
  Files             ?      305           
  Lines             ?    13103           
  Branches          ?        0           
=========================================
  Hits              ?    12000           
  Misses            ?     1103           
  Partials          ?        0

Impacted Files	Coverage Δ	Complexity Δ
Spanner/src/TransactionConfigurationTrait.php	100% <100%> (ø)	24 <0> (?)
Spanner/src/Database.php	100% <100%> (ø)	74 <0> (?)

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