Safer and more efficient healing

[ArneBab]

With the healing changes in 1497 there’s much more healing traffic for large files. But that brings new risks with it.

This PR tries to address them.

 * Specialize Healing to the fraction of the keyspace in which we would receive the inserts
 * if we were one of 5 long distance nodes of an actual inserter.
 *
 * If an opennet node is connected to an attacker, healing traffic could be mistaken for an insert.
 * Since opennet cannot be fully secured, this should be avoided.
 * As a solution, we specialize healing inserts to the inserts we would send if we were one of 5 
 * long distance connections for a node in another part of the keyspace.
 *
 * As a welcome side effect, specialized healing inserts should take one hop less to reach the
 * correct node from which loop detection will stop the insert long before HTL reaches zero.

[ArneBab]

Merged — thanks a lot for your reviews!