Compile DatabaseWriter for CVMFS Builds

LCG_101 ships libpqxx which means we can enable the BUILD_ALL_MODULES flag and build every possible Allpix Squared module in the pipeline and for the CVMFS deployments. A few fixes have been necessary:

• LCG_101 has libpqxx version 7.x while Ubuntu ships 6.x. Unfortunately the pqxx::connection interface changed, forcing us to include an #if.
• One minor complaint from Clang. I even thought about turning off that warning, but fixed it instead. 😼