circuit_unlink_all_from_orconn is too expensive
Moritz's relay spends a lot of time in circuit_unlink_all_from_orconn, which does a linear walk over the entire circuit list.
To make this faster, we'll need data structures. Currently, each or_connection has a doubly-linked list of its active circuits, but not of all its circuits. Adding such a doubly-linked-list is the obvious solution.
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