perf: withstand connect storms

[olavloite]

Use a background worker thread pool to create new connections. This reduces the probability that the client will timeout while waiting for a connection, when a large number of connections are being requested at the same time.

Fixes #1377

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (0a78cf6) 90.66% compared to head (4493cc5) 90.63%.
Report is 3 commits behind head on postgresql-dialect.

Files	Patch %	Lines
...om/google/cloud/spanner/pgadapter/ProxyServer.java	72.72%	3 Missing ⚠️
...gle/cloud/spanner/pgadapter/ConnectionHandler.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                   Coverage Diff                    @@
##             postgresql-dialect    #1414      +/-   ##
========================================================
- Coverage                 90.66%   90.63%   -0.03%     
- Complexity                 2640     2642       +2     
========================================================
  Files                       141      141              
  Lines                      9016     9030      +14     
  Branches                   1313     1314       +1     
========================================================
+ Hits                       8174     8184      +10     
- Misses                      567      570       +3     
- Partials                    275      276       +1     

Flag	Coverage Δ
all_tests	90.63% <73.33%> (-0.03%)	⬇️

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