feat(spanner): send session-refresh request at low priority

[devbww]

Use low refresh priority for the "SELECT 1" "send and forget"
session-refresh query.

Also add a note about the default transaction mode used for
that query.

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: c3a50a4ea089138c4d26880dc965ce63aa571c8a

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[codecov]

Codecov Report

Merging #7140 (c3a50a4) into main (d009917) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7140   +/-   ##
=======================================
  Coverage   94.49%   94.49%           
=======================================
  Files        1310     1310           
  Lines      113146   113147    +1     
=======================================
+ Hits       106912   106916    +4     
+ Misses       6234     6231    -3     

Impacted Files	Coverage Δ
google/cloud/spanner/internal/session_pool.cc	91.25% <100.00%> (+0.03%)	⬆️
google/cloud/pubsub/subscriber_connection_test.cc	97.20% <0.00%> (-0.70%)	⬇️
.../cloud/storage/benchmarks/throughput_experiment.cc	74.87% <0.00%> (+0.50%)	⬆️
google/cloud/storage/internal/curl_handle.h	82.85% <0.00%> (+2.85%)	⬆️
google/cloud/bigtable/internal/common_client.cc	100.00% <0.00%> (+2.85%)	⬆️

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