fixed connectionpool concurrency test:

- reduce aquired connections to 80 since postgresql has 100 max connections per default
- added additional assertion for total number of aquired connections
fixed connection checking

can't use lastUse to determine if connection needs to be tested since it's updated
on connection release. introduced lastCheck instead.

additionally try to close the connection if invalid to free any bound resources, and
fixed timeout for isValid() - it's seconds, not millis
surround connection retrieval in try/catch block to keep running even if no connection can be
retrieved (eg. in case the database is down)