deprecation: deprecate MaxIdleSessions configuration option

[olavloite]

Deprecates the MaxIdleSessions configuration option for the SessionPool. Users should instead set a higher value for MinSessions instead of using MaxIdleSessions. This will reduce both the complexity of session pool configuration and the session pool maintenance.

Fixes #865

[codecov]

Codecov Report

Merging #926 (13c12f4) into master (ec99cc7) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #926   +/-   ##
=========================================
  Coverage     85.22%   85.22%           
- Complexity     2651     2652    +1     
=========================================
  Files           145      145           
  Lines         14358    14358           
  Branches       1391     1391           
=========================================
+ Hits          12236    12237    +1     
  Misses         1539     1539           
+ Partials        583      582    -1     

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <ø> (ø)	18.00 <0.00> (ø)
...m/google/cloud/spanner/connection/SpannerPool.java	87.36% <0.00%> (-0.53%)	33.00% <0.00%> (ø%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.26% <0.00%> (+0.19%)	73.00% <0.00%> (+1.00%)

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