Allow querying accounts touched within the block

[MaksymZavershynskyi]

We should extend RPC API to query all accounts that were touched within the given block.

[frol]

The implementation is already there, account_id that is specified is used as a prefix, so passing an empty account_id works as one requested in this issue. This is definitely unexpected, so I will work on the design a little bit more, and make this feature clear especially since it is not really expected to receive info about changes to vlad, vlad111 and vlad222 accounts when you specify vlad as account_id and expect only the exact match to be filtered.

[frol]

I was under a wrong impression previously. Well, it was a bug turning as a feature. I will need to implement a new RPC endpoint exposing a list of account IDs touched in a block.

Draft API:

$ http post http://localhost:3030 jsonrpc=2.0 id=dontcare \
    method=EXPERIMENTAL_changes_list_accounts \
    'params:={"block_id": "DYHRzRbxUR1ANPdCQcgQE9g5zyYQoDZ1k8BJEQ3hDSgW"}'

$ http post http://localhost:3030 jsonrpc=2.0 id=dontcare \
    method=EXPERIMENTAL_changes_list_accounts \
    'params:={"block_id": 100500}'

$ http post http://localhost:3030 jsonrpc=2.0 id=dontcare \
    method=EXPERIMENTAL_changes_list_accounts \
    'params:={"finality": "latest"}'

NOTE: The endpoint naming needs some bikeshedding.

The response:

{
    "block_hash": "DYHRzRbxUR1ANPdCQcgQE9g5zyYQoDZ1k8BJEQ3hDSgW",
    "changes": [
        {
            "account_id": "frol.near",
            "type": "data_changes",
        },
        {
            "account_id": "frol.near",
            "type": "account_changes",
        },
        {
            "account_id": "user.near",
            "type": "access_key_changes",
        },
    ]
}

[frol]

The follow-up documentation request is near/docs#267