agent: Continue to retry indefinitely

[nemunaire]

When the woodpecker server is not reachable (eg. for update, maintenance, agent connection issue, ...) for a long period of time, the agent tries continuously to reconnect, without any delay. This creates several GB of logs in a short period of time.

Here is a sample line, repeated indefinitely:

{"level":"error","error":"rpc error: code = Unavailable desc = connection error: desc = \"transport: Error while dialing: dial tcp x.x.x.x:xxx: connect: connection refused\"","time":"2024-04-07T17:29:59Z","message":"grpc error: done(): code: Unavailable"}

It appears that the backoff package, after a certain amount of time, returns backoff.Stop (-1) instead of a valid delay to wait. It means that no more retry should be made, as shown in the example. But the code doesn't handle that case and takes -1 as the next delay.
This led to continuous retry with no delay between them and creates a huge amount of logs.

MaxElapsedTime default is 15 minutes, passed this time, NextBackOff returns backoff.Stop (-1) instead of MaxInterval.
This commit sets MaxElapsedTime to 0, to avoid Stop return.

[lafriks]

Failure unrelated to this PR

[woodpecker-bot]

Tearing down https://woodpecker-ci-woodpecker-pr-3599.surge.sh