ULC: mechanism to remove and add trusted servers

[JekaMas]

At the moment we can't ban for a period of time any trusted server that sends incorrect or malicious blocks. We should have one.

And we should have a possibility to extend a list of trusted servers: explore new servers with good stats, validate them and add to the trusted list.

Both parts need some storage to store data related to trusted nodes stats and blacklist information.

Implementation steps:

• Store trusted nodes list in the DB
• Store stats: number of incorrect blocks since last epoch
• Store blacklist data: when a blacklisted node should be removed from the list
• Load trusted nodes data from the DB. If DB doesn't contain any data, we should use config data
• Add config param, when we should add a node to the blacklist, how long should it be.
• Remove nodes from the blacklist after given period of time
• Adding new trusted nodes mechanism: for example if an untrusted server sends to a user correct blocks for an epoch, we can trust this node and add it to the list.
• Add config param to disable adding new trusted nodes mechanism