Add the ability to remove added Cores

[Marko]

User should be able to remove any Core added. When Core is removed, it should be also remembered and blocked in the future.

Replication already has the ability to select what Cores the user wants to share and receive. Combined with persisted block list in the meta core we can easily remove blocked cores from both want and share lists on the fly.

---

Deliverables

• user can use remove_core(core_key) to remove a core
• core is removed from meta core
• core is automatically removed from want list in replication
• core is automatically removed from sharing list in replication
• core is remembered as blocked