Deactivate connection killer by default

The connection killer kills connections after a configurable delay, to avoid leaks in case some runaways process never returns it and never interacts with the CoreSession (which would provoke a transaction timeout).

However the connection killer process is inherently flawed because it runs from a separate thread and is not able to cleanup everything in the connection pool, because the connection pool has thread-related caching datastructures. This results connection returned to the pool when they are killed but in an unusable state because they have not correctly been cleaned up.

So we should deactivate the connection killer by default, before removing it entirely in a further release.