Add a SQL busy handler with 2 seconds timeout

[lbonn]

We've talked about it but it got forgotten somehow.
It's maybe not required for our current use but I figured it should not hurt.

[pattivacek]

Seems like it can't hurt. Am I reading the docs right that the default is just to fail immediately if you don't set this?

[codecov-io]

Codecov Report

Merging #1648 into master will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1648      +/-   ##
==========================================
- Coverage   82.72%   82.61%   -0.11%     
==========================================
  Files         191      191              
  Lines       12061    12062       +1     
==========================================
- Hits         9977     9965      -12     
- Misses       2084     2097      +13     

Impacted Files	Coverage Δ
src/libaktualizr/storage/sql_utils.h	86.07% <100.00%> (+0.17%)	⬆️
src/libaktualizr/package_manager/ostreemanager.cc	79.04% <0.00%> (-1.48%)	⬇️
src/libaktualizr/storage/sqlstorage.cc	77.40% <0.00%> (-0.94%)	⬇️
src/aktualizr_primary/secondary.cc	86.95% <0.00%> (-0.87%)	⬇️
src/libaktualizr/storage/sqlstorage_base.cc	81.20% <0.00%> (+1.34%)	⬆️

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