Fix tree updates iterator

[sergeypospelov]

Fix a problem, found by @tochilinak, in the tree updates iterator.

Problem

Suppose, you have such writes to the memory region:

{k1 <- v1}{k2 <- v2}{k1 <- v1}

The latter update {k1 <- v1} will be filtered out, due to emittedUpdates hash set. This is because UPinpountUpdate has custom equals and hashcode based on guard and key equality which is incorrect. Now, updates have reference equality equals.