Improve the handling of dirty paths and mutation locking in the server

[ljeub-pometry]

Right now, reading a path cleans up any folders marked as dirty, this means that reads need to hold a read-lock on the mutations. Otherwise, a read might clean up an in-progress path used by a mutation. This makes it very easy to run into accidental dead locks, see #2787.

We should change this such that:

• reads do not clean up dirty paths and treat any path marked as dirty the same way as a path that doesn't exist
• mutations only hold an upgradeable read-lock initially while they resolve the path until they have written the dirty marker at which point they downgrade the lock to a normal read-lock. Only if part of the path for the mutation is marked dirty or the mutation needs to create a dirty marker in a folder that already contains a marker file, the lock is upgraded to a write lock. With the write lock any dirty path can be cleaned up as needed (no other mutation in progress).