Roll buffer files on size/limit total disk usage in durable mode

[nblumhardt]

#52 highlights some problems in per-day buffer files when log volumes are high.

This PR takes advantage of the new functionality in Serilog.Sinks.File 4.0 to roll buffer files on 100 MB boundaries, so that space can be reclaimed within the one day period.

The PR is breaking - slightly - in that the parameter previously called bufferFileSizeLimitBytes has been renamed to bufferSizeLimitBytes and is applied to the whole file set instead of individual files. By doing this, we can better control disk usage, and ensure the oldest data is discarded when the limit is reached.

    .WriteTo.Seq("http://localhost:5341",
                 bufferBaseFilename: "logs/buffer",
                 bufferSizeLimitBytes: 1_000_000_000)

In practice the change in semantics shouldn't have a huge negative impact - the behavior upon hitting the old limit was undesirable, so the value is usually set high.