Limit the number of sinks held open in the map

[nblumhardt]

More scenarios would be supported here if "old" sinks could be automatically garbage collected/removed from the map.

For example, to emulate rolling files, but across rolling directories:

WriteTo.Map(
    le => le.Timestamp.Date,
    (date, lc) => lc.WriteTo.File($"./logs/{date:yyyyMMdd}/log.txt"))

Via: https://stackoverflow.com/questions/44501377/serilog-rollingfile-sinks-create-folder-named-with-date