front- & backend synchronization issues for document operations resutling in 404

[duncdrum]

see #516

the (asynchronous) rendering of the tree items for documents (and very likely collections) and the corresponding backend operations are often not in sync.

Files displayed in the frontend, no longer exist on the backend, and vice versa.
Which leads to frequent 404 when running our testsuite, but can easily be reproduced manually as well, see #433, #517

GET http://localhost:8080/exist/restxq/fusiondb/index?uri=/db 404 (Not Found)

Note that in the above /db is supposedly not available which is pretty much impossible.
we need to reconsider the current design, and come up with something less flaky and more stable.