Genesis epoch for bottom-up checkpoints

[adlrocha]

Currently we use genesis_epoch = 0 by default in all subnets:

ipc-actors/subnet-actor/src/lib.rs

Line 89 in 5ef2203

let genesis_epoch = 0;

The reason for this is that we assume that all subnet will be spawned with epoch 0, and thus, new bottom-up checkpoints must be submitted from the subnet's genesis. This is true for now, but once we support the docking of IPC-compatible subnets, or subnets changing parents, we will need to set the genesis epoch to the first epoch where the network docked to the new parent instead of 0. This way, the subnet actor doesn't require the commitment from every checkpoint from genesis to be able to execute from the moment it docked to the new parent.

Having genesis_epoch != 0 will only be needed when we allow a subnet to become a child from an IPC parent when it is already mining blocks to avoid having to submit every checkpoint from zero. This is a consequence of the new design for checkpoint voting, where we don't allow gaps between checkpoints to ensure that all checkpoints (and the underlying cross-net messages included) are executed.