ARUHA-97 Tune connection pool
ARUHA-97 Tune connection pool

There are currently 2 major issues with the connection pool on the test
environment:

1. It tries to open too many connections with the database
2. It does not recover from a database restart (or a network problem)

In order to fix those issues, we are adding some checks and also
avoiding requesting too many connections during application boot.

zalando/nakadi,zalando/nakadi