Allow connecting to multiple kernel gateway servers

When we connect to kernel gateway, we loose access to local kernels and we can connect to only one kernel gateway at a time. One possible way to address this could be adding something like a `UnionKernelManager` which will take a list of objects implementing `MappingKernelManagers`. In case of conflicts/duplicates, the we can solve the kernel manager to use based on priority order.

Start by tracing how kernel gateway connections and local kernels are currently represented through MappingKernelManagers. Evaluate the proposed UnionKernelManager, including priority-based handling of duplicate kernels across multiple managers; done means local kernels and multiple gateway servers can be accessed together without ambiguous selection.

Written by the indexing model from the issue text.