backoff: fix enforcing max backoff

[andydunstall]

The Piko client uses backoff with jitter to avoid overloading the server when reconnecting, however the client wasn't enforcing max backoff meaning it could grow far too large...

[moredure]

lgtm

[andydunstall]

Also updated to log how long the backoff is (otherwise it's quite hard to debug):

{"level":"warn","ts":"2024-12-05T10:35:31.167Z","subsystem":"client","msg":"connect failed; retrying","endpoint-id":"my-endpoint","url":"ws://localhost:8001/piko/v1/upstream/my-endpoint","backoff":"109.574759ms","error":"dial tcp 127.0.0.1:8001: connect: connection refused"}
{"level":"warn","ts":"2024-12-05T10:35:31.278Z","subsystem":"client","msg":"connect failed; retrying","endpoint-id":"my-endpoint","url":"ws://localhost:8001/piko/v1/upstream/my-endpoint","backoff":"228.701585ms","error":"dial tcp 127.0.0.1:8001: connect: connection refused"}
{"level":"warn","ts":"2024-12-05T10:35:31.509Z","subsystem":"client","msg":"connect failed; retrying","endpoint-id":"my-endpoint","url":"ws://localhost:8001/piko/v1/upstream/my-endpoint","backoff":"488.785139ms","error":"dial tcp 127.0.0.1:8001: connect: connection refused"}
{"level":"warn","ts":"2024-12-05T10:35:31.999Z","subsystem":"client","msg":"connect failed; retrying","endpoint-id":"my-endpoint","url":"ws://localhost:8001/piko/v1/upstream/my-endpoint","backoff":"1.041538359s","error":"dial tcp 127.0.0.1:8001: connect: connection refused"}
{"level":"warn","ts":"2024-12-05T10:35:33.042Z","subsystem":"client","msg":"connect failed; retrying","endpoint-id":"my-endpoint","url":"ws://localhost:8001/piko/v1/upstream/my-endpoint","backoff":"2.278453609s","error":"dial tcp 127.0.0.1:8001: connect: connection refused"}
{"level":"warn","ts":"2024-12-05T10:35:35.324Z","subsystem":"client","msg":"connect failed; retrying","endpoint-id":"my-endpoint","url":"ws://localhost:8001/piko/v1/upstream/my-endpoint","backoff":"4.77646657s","error":"dial tcp 127.0.0.1:8001: connect: connection refused"}