Use an upgradable pattern for the external modules

[akolotov]

Currently there are three modules serving some aspects of the bridge contracts configuration:

• the fee manager
• the gas limit manager
• the forwarding rules manager

These modules are implemented as a separate contracts and the bridge contract calls the modules' methods to get/store some information.

During the bridge usage the modules could collect lots of configuration data. But if an issue is found in the code of the modules they need to be updated. Since current contracts of the modules do not support the upgradable pattern, it means that the bridge admins could be required to perform lots of transactions to migrate the state from the old version of the module to the new one.