Make gateway upgrade process faster in case of breaking node upgrade

[lumtis]

When the blockchain node upgrade and require a contract upgrade because of breaking interface the following problem can happen:

• The upgrade is scheduled for a height
• Once height is reached, every gateways need be upgraded. The upgrade is a sensitive action using a setup with maximum number of signers. The upgrade can be a lengthy process but would have to happen as soon as upgrade height is reached.

Proposed solution

Upgrade is sensitive because contract can be replaced by a malicious one, not because of the upgrade itself. The process could be made more flexible to prepare for the upgrade.

• A new role is introduced: UPGRADE_EXECUTOR, requires less signatures than upgrade admin
• Upgrade admin propose new implementation contract to be schedule
• The upgrade executor can execute the upgrade to this implementation at any time