Implement incremental recovery

Currently BTM requires that all connection pools be created before startup.

This is cumbersome and could cause some problems when not all resources are available during startup recovery, like denial of startup.

Incremental recovery would make the recovery process more flexible; resources would be recovered independently when they are registered.

This will require changes in the disk logger and the recovery process. It might also require a change in the logs format but that should be avoided if possible.