Start new ct-watchers as new ct logs become available

[d-Rickyy-b]

Currently new logs are not watched as they become available, because we only import the current log list after starting certstream-server-go. In a case where it's running for months at a time, we might miss newer logs and hence miss certificates.

We also might need a way to properly identify that a cert log was shut down. Currently there's a retry mechanism in place that restarts existing ct watchers if any errors occurr.

Originally asked by @uggyuggy in #41