* Change default log config to buffer by default.

This batches up writes to the filesystem, which is more efficient for
disk I/O. This means that it can take some time for logs to get written
to disk. Note that ERROR logs (and above) immediately flush the buffer.

This only effects new installs, as we only write the log config if
started with `--generate-config` (in the same way we do for generating
signing keys).

* Default to keeping last 4 days of logs.

This hopefully reduces the amount of logs kept for new servers. Keeping
the last 1GB of logs is likely overkill for new servers, but equally may
not be enough for busy ones.

Instead, we keep the last four days worth of logs, enough so that admins
can investigate any problems that happened over e.g. a long weekend.