feat: persist data to disk

[agaffney]

Fixes #6

[agaffney]

$ ./chnsd 
{"level":"info","timestamp":"2023-08-08T17:45:20-05:00","caller":"indexer/indexer.go:81","msg":"starting new chainsync at configured location: 34255089, a35a0d216e00c64c257e41089138f4d22be721281d73f2ab8ef61ca8863d04a0"}
{"level":"info","timestamp":"2023-08-08T17:45:21-05:00","caller":"chnsd/main.go:72","msg":"starting DNS listener on :8053"}
{"level":"info","timestamp":"2023-08-08T17:45:21-05:00","caller":"indexer/indexer.go:154","msg":"found updated registration for domain: snorkel.cardano"}
{"level":"info","timestamp":"2023-08-08T17:45:24-05:00","caller":"indexer/indexer.go:154","msg":"found updated registration for domain: dolphin.cardano"}
^C
$ ./chnsd 
{"level":"info","timestamp":"2023-08-08T17:45:34-05:00","caller":"indexer/indexer.go:64","msg":"found previous chainsync cursor: 34255785, 0acafd824cef0283cb94a62a0e77c7d0ca88c23f53ee5c74e512ff38c14321f8"}
{"level":"info","timestamp":"2023-08-08T17:45:34-05:00","caller":"chnsd/main.go:72","msg":"starting DNS listener on :8053"}

[wolf31o2]

This will dump a state directory wherever this is executed. This is fine, for now, but we really should put this somewhere better... ${HOME}/.cache/chnsd/state?

[agaffney]

I'm definitely open to a better default location for this