- Use the default options CREATE, TRUNCATE_EXISTING, and WRITE for
   overwriting the state file rather than just CREATE. The effect was
   that we kept lines from before, which is not a big issue, but also
   not what we wanted.

 - Avoid duplicating code from WebServerAccessLogPersistence for
   calculating storage paths.

Web servers typically provide us with the last 14 days of request
logs. We shouldn't process the whole 14 days over and over. Instead we
should only process new logs files and any other log files containing
log lines from newly written dates.

In some cases web servers stop serving a given virtual host or stop
acting as web server at all. However, in these cases we're left with
14 days of logs per virtual host. Ideally, these logs would get
cleaned up, but until that's the case, we should at least not
reprocess these files over and over.

In order to avoid reprocessing webstats files, we need a new state
file with log dates contained in given input files. We use that state
file to determine which of the previously processed webstats files to
re-process, so that we can write complete daily logs.

Temporary commit: requires update to metrics-base commit 264e498 as
soon as that's merged to master.

 - Remove package-internal abstract class.

 - Remove package-internal interfaces InternalLogDescriptor and
   InternalWebServerAccessLog.

 - Remove unused code.

 - Copy types from metrics-lib to this code base.
 - Update package and import statements.
 - Copy remaining parts of metrics-lib's FileType.

The only functionality contained in metrics-lib's internal package is
file (de-)compression, which in turn uses a third-party library that
we're using anyway. This is a weak reason for depending on our own
library for this functionality. Removing this dependency will make it
easier to make changes to our library in the future.

The new FileType class is based on a copy of the same enum type in
metrics-lib without @since tags and without methods that we don't use.