Added TreeNodeLoadStrategy interface

This was part of TU7.2 task to make tree node load itself.
I decided that the idea of making tree node load itself was not very good,.
but TreeNodeLoadStrategy is a good thing
Deleted `TreeNodeLoadStrategy` interface

... and `AllTimeTreeNodeLoadStrategy` implementation.

TU7.6 task is discarded.

Who loads the tree, knows its implementation. This is why it should also be responsible for implementing
"tree node mover". It's `ActivityTreeManager` who does both know. I cannot move responsibility for loading
tree into `TreeNodeLoadStrategy`.
TU7.5: Load activity tree without accessing nodes by id in `HashMap`.