Make enclave extrinsics mortal

[brenzi]

closes #1475

approach:

• separate mortality from tip
• extend ParentchainCall type to include mortality info for each call separately
• use parentchain last finalized header info wherever we can get it from, depending on context: either ocall or parentchain pallets
• fall back to immortal, if last finalized header can't be obtained (is this dangerous?)

critical:

• Stf-triggered extrinsics, most important: balance_unshield

good to have to avoid jamming the chain after downtimes:

• enclaveBridge.confirmProcessedParentchainBlock
• sidechain.FinalizedSidechainBlock
• attestation

only needed for init, low prio

• shard vault creation
• shard config

[clangenb]

Looks good so far!

[clangenb]

Looks good to me! Only one remark:

• fall back to immortal, if last finalized header can't be obtained (is this dangerous?)

I don't think it is dangerous, the question is rather if it is realistic that the rest of the service if up and running well if we can't obtain it. I think the only possible scenario is if the parentchain is having finalization issues right at the time when the worker is starting up. Hence, we might consider falling back in this order: finalized head -> best head -> immortal. Not sure if it is worth implementing though.