Remove already executed cross-messages from gateway actor state

[adlrocha]

Related: consensus-shipyard/ipc-actors#20

We currently don't have a way to signal that a cross-net message has been successfully executed and that can be removed from the queue of cross-net messages in the corresponding gateway. This will generate the state in the gateway to increase indefinitely (and unnecessarily), increasing the gas costs of new cross-net messages, as it will keep expanding the memory requirements of the gateway.

• For top-down messages, once the message is executed in the child subnet, the parent signal should be signalled to notify that the corresponding message can be removed from top_down_msgs for the subnet: https://github.com/consensus-shipyard/ipc-actors/blob/ec4dbe39d21c0ab3b29fd3f5f5f8d5edc7f50d93/gateway/src/subnet.rs#L28. Parents never sync with their children so we need some participant in the child to propagate the signal up to the parent. One way could be to include in the checkpoint information about all the cross-messages executed and that can be cleaned from this data structure.
• For bottom-up messages, the only piece of information persisted in the gateway of the parent is the cross-message meta: https://github.com/consensus-shipyard/ipc-actors/blob/main/gateway/src/state.rs#L47. Executed crossMsgMeta can be more easily cleaned, as when the messages included in it are executed, the corresponding crossMsgMeta could be conveniently cleaned.
• send_cross propagated messages are already being conveniently removed from the postbox: https://github.com/consensus-shipyard/ipc-actors/blob/ec4dbe39d21c0ab3b29fd3f5f5f8d5edc7f50d93/gateway/src/lib.rs#L857

Implementation

• Top-down messages: To garbage collect top-down messages, we can propagate in checkpoints the latest nonce of the top-down message execute in the child subnet. The parent will pick up this nonce and garbage collect all the message below and including that nonce from its state.
• Bottom-up messages: For bottom up messages it is easier. When a bottom up message for a new nonce is executed, the message meta for that nonce can be directly cleaned from the parent's state.