MINOR: ebtree: add new eb_next_dup/eb_prev_dup() functions to visit duplicates

Sometimes it's very useful to visit duplicates of a same node, but doing
so from the application is not convenient because keys have to be compared,
while all the information is available during the next/prev steps.

Let's introduce a couple of new eb_next_dup/eb_prev_dup functions to visit
only duplicates of the current node and return NULL once it's done. Now we
have all 3 combinations :
  - next        : returns next node in the tree
  - next_dup    : returns next dup in the sub-tree
  - next_unique : returns next value after skipping dups

(cherry picked from commit 3327b8ae6866f3878322a1a29e70b450226d216d)