Enable socket keepalive on MongoDB connection

The MongoDB connection can be tuned using the URI option.

But some options are not available from the URI, this is the case for the socketKeepalive option that should be activated by default. This can help to prevent firewall to disconnect inactive connection.

Also by default we should not wait for ever on connect and socket read operations.

Default option is connectTimeoutMS to 30s and socketTimeoutMS to 60s, can be adjusted using the URI option.