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.
