This commit introduces some helpers so that ETags can be calculated at
the same time that files get read or written. Besides looking nicer, it
should also help reduce lock contention around file access, as files do
not need to be opened twice anymore.
This commit adds read/write locking for individual files, so that
concurrent requests (e.g. to read and write the same file) cannot
interfere with one another.
The locking is not very fine-grained at the moment, and can probably be
improved upon. But it does ensure consistency.
Thanks to the latest version of go-webdav, this is now a thing. A lot of
operations (like creating a calendar) are not yet supported. But the
basics work fine. Note that multi-calendar means that different users
can each have their own calenders. Resource sharing is not yet
implemented either.
Includes the adding of a lot of debug logs, as issues are otherwise
pretty hard to figure out. The logging still needs to be made more
consistent, and probably cleaned up a bit in some places.
