Add CLCACHE_RECACHE feature

[oktal3700]

Just like in ccache, this puts clcache into a "write-only" mode.
Objects will be stored in the cache, but objects already in the cache
will not be reused. The real compiler is always used.

This is useful in CI when you have a release branch that you want to
always use the real compiler, and feature branches that you want to
reap the benefit of the cache generated from the release branch.
In that case you would enable CLCACHE_RECACHE only when building from
the release branch.

[codecov-io]

Codecov Report

Merging #348 into master will increase coverage by 0.36%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #348      +/-   ##
==========================================
+ Coverage   88.93%   89.29%   +0.36%     
==========================================
  Files           4        4              
  Lines        1301     1308       +7     
  Branches      195      199       +4     
==========================================
+ Hits         1157     1168      +11     
+ Misses        106      105       -1     
+ Partials       38       35       -3

Flag	Coverage Δ
#integrationtests_memcached	67.24% <14.28%> (-0.38%)	⬇️
#unittests	86.23% <71.42%> (+0.38%)	⬆️

Impacted Files	Coverage Δ
clcache/__main__.py	91.28% <85.71%> (+0.4%)	⬆️

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[frerich]

Looks good to me -- just a test seems to be missing. :-)

[oktal3700]

Added a testcase.