Add token_network_settings and channel_settings tables to database

[lullis]

As part of the #5011, we should be able to store user-defined values for some of the settings parameters. Non-exhaustive list of examples:

• FeeSchedule per Channel
• FeeSchedule per TokenNetwork
• List of Matrix Services that the node should try to connect
• Minimum REI Balance for node operation
• Maximum Gas Price/Gas Limit
• Maximum fee to pay to PFS
• Transport Parameters

Specification

Some of these settings are related to the operation of the node and we can use the already-existing settings table on sqlite, which currently only stores the version record. There are however some parameters which can and should be specified per token network and/or channel, with the value specificed on settings.py only to be used as a default. So the proposal is to create the following tables:

CREATE TABLE token_network_settings (
    chain_id int NOT NULL,
    token_network_id VARCHAR[32] NOT NULL,
    name VARCHAR[24],
    value TEXT,
   UNIQUE(chain_id, token_network_id, name)
);

Likewise for per-channel settings:

CREATE TABLE token_network_settings (
    chain_id int NOT NULL,
    token_network_id VARCHAR[32] NOT NULL,
    channel_id VARCHAR[32] NOT NULL,
    name VARCHAR[24],
    value TEXT,
   UNIQUE(chain_id, token_network_id, chain_id, name)
);

Backwards Compatibility

Previous versions should not be affected by these new tables.

[lullis]

I am not really sure channel_settings will be needed after all. We can keep token_network settings as part of the state, and using the settings tables for configuring/storing the default values to be used as initial values.