Remove RefreshDB and related code from SessionDB

[tpazderka]

• Any changes relevant to users are recorded in the CHANGELOG.md.
• The documentation has been updated, if necessary.
• New code is annotated.
• Changes are covered by tests.

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[codecov-io]

Codecov Report

Merging #674 into master will increase coverage by 0.14%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #674      +/-   ##
==========================================
+ Coverage    62.5%   62.64%   +0.14%     
==========================================
  Files          62       62              
  Lines       11382    11307      -75     
  Branches     2000     1988      -12     
==========================================
- Hits         7114     7083      -31     
+ Misses       3693     3656      -37     
+ Partials      575      568       -7

Impacted Files	Coverage Δ
src/oic/utils/sdb.py	86.07% <0%> (+5.76%)	⬆️
src/oic/utils/http_util.py	68.53% <0%> (+0.56%)	⬆️
src/oic/utils/authn/user.py	64.22% <0%> (+0.81%)	⬆️

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