Adds configuration options for database directory, either on command line through (--db dir-name) or the config.toml file. Fixes: https://todo.sr.ht/~gheartsfield/nostr-rs-relay/13
A configuration option, `messages_per_sec`, imposes a global limit on the rate for which new events can be stored. Fixes https://todo.sr.ht/~gheartsfield/nostr-rs-relay/6