Set real limits for deserialization maximum values

[AurelienFT]

When deserializing, we need to make sure that everything has limits to avoid a message to break the memory of the node. We still have some variables that don't have a limit in mind :

• [x] MAX_LEDGER_CHANGES_PER_BLOCK. We also need to apply this limit in block execution.
• [ ] MAX_DATASTORE_ENTRY_COUNT. We also need to apply this limit in the code. Possible values : > 50000
• [ ] Make the type of the constants standard. Possible : u64

[AurelienFT]

@damip can we take decisions on that ? I think that's nice if this is resolve before the tests.

[AurelienFT]

Now that we launched the mainnet reducing the maximum size of element in a datastore is a breaking change and should be done carefully.