BUG/MEDIUM: acme: move from mt_list to a rwlock + ebmbtree

BUG/MEDIUM: acme: move from mt_list to a rwlock + ebmbtree

The current ACME scheduler suffers from problems due to the way the
tasks are stored:

- MT_LIST are not scalables when having a lot of ACME tasks and having
  to look for a specific one.
- the acme_task pointer was stored in the ckch_store in order to not
  passing through the whole list. But a ckch_store can be updated and
  the pointer lost in the previous one.
- when a task fails, the ptr in the ckch_store was not removed because
  we only work with a copy of the original ckch_store, it would need to
  lock the ckchs_tree and remove this pointer.

This patch fixes the issues by removing the MT_LIST-based architecture,
and replacing it by a simple ebmbtree + rwlock design.

The pointer to the task is not stored anymore in the ckch_store, but
instead it is stored in the acme_tasks tree. Finding a task is done by
doing a lookup on this tree with a RDLOCK.
Instead of checking if store->acme_task is not NULL, a lookup is also
done.

This allow to remove the stuck "acme_task" pointer in the store, which
was preventing to restart an acme task when the previous failed for this
specific certificate.

Must be backported in 3.2.