Limit the number of sessions (and their kernels) created at a single scheduler tick

[fregataa]

When there are a large number of scheduled sessions for a single scheduler tick, making all RPC calls at once may cause the manager performance issues and potential hangs.

Let's apply the concept of more_itertools.chunked() when making the RPC calls for each kernel creation, introducing a new configuration option to specify the chunk size.

We should also check if the hang happens when there are overlapping RPC calls to the same agent, even when there is a low number of kernels to create.