GGP-0034: Enable permissionless payers

GIP: 0056 Title: Permissionless payers Authors: Zac Burns

This proposal removes the limitation of specific governance-approved entities to pay query fees to Indexers in the network.

The code changes remove the assetHolders mapping from the staking contract and all associated functions, checks, and events, and implements some additional protections to perform this change securely.

Upon a successful vote of this proposal, the latest audited implementation of the change (https://github.com/graphprotocol/contracts/pull/864 plus related audit fixes) will be deployed by the Edge & Node Smart Contracts team following a test and deployment plan.