Connection Pool:
- Connection pools (#499)
- Connection pools documentation and example (#533)

Smaller features:
- Add connection methods is_connected() and ping_server() (#528)
- Add sqlpp::compat::make_unique (#527)

Cleanup:
- Multiple code cleanups, in particular date/time code (e.g. #509, #511, #514, #517, #520)
- Replace include guards with "#pragma once" (#506)