Make tasks local only.

[gui1117]

Related to #9693

In the transaction pool, transaction are identified by the tag they provide.

For tasks the provided tag is simply the hash for the encoded task.

Nothing in the doc says that implementers should be careful that tasks are not too many for a single operation. What I mean is if a task is migrate_keys(limit), with valid first from 1 to 10_000. Then all tasks migrate_keys(1), migrate_keys(2) ... migrate_keys(10_000) are valid and effectively do the same operation: they all migrate part of the keys.
In this case a malicious person can submit all those tasks at once and spam the transaction pool with 10_000 transactions.

I see multiple solution:

• (1) we are careful when we implement tasks, we make the doc clear, but the API is error prone. (in my example above we would implement just migrate_keys and inside the call we would do a basic rate of migration of 1000 keys in a bulk).
• (2) we have a new value returned that is the provided tag for the task. Or we use the task index as provided tag.
• (3) we only accept local tasks: <-- implemented in this PR.

maybe (2) is a better API if we want external submission of tasks.