fix: enforce load limits when loading snapshot

[romange]

Prevent loading snapshots with used memory higher than max memory limit.

1. Store the used memory metadata only inside the summary file
2. Load the summary file before loading anything else, and if the used-memory is higher, abort the load.

[adiholden]

What about dfly load append command? you will not flush the database before you load the snapshot file. If we want to enforce this we need to check current_used_memory + snapshot used memory < max_memory_limit

[romange]

This command is not interesting as is only used for migrations which are ad hoc at this point.