Re-use SQL connections per default

[sebastian-peter]

With #272, an option to reuse an existing SQL connection within SqlConnector was provided. The default method getConnection() still creates a new connection with every call though, which can lead to the maximum connections limit (e.g. with Postgres) to be exceeded.
Re-using existing connections does not seem to have any disadvantages for single-thread usage of SqlConnector. If multiple threads access the same SqlConnector, this might differ and getConnection(false) should be used.