BUG/MINOR: peers: Report a resync was explicitly requested from a thread-safe manner

BUG/MINOR: peers: Report a resync was explicitly requested from a thread-safe manner

Flags on the peers section state must be updated from a thread-safe manner.
It is not true today. With this patch we take care PEERS_F_RESYNC_REQUESTED
flag is only set by the resync task. To do so, a peer flag is used. This
flag is only set once and never removed. It is juste used for debugging
purpose. So it is enough to set it on a peer and be sure to report it on the
peers section when the sync task is executed.

This patch relies on previous ones:

 * MINOR: peers: Add functions to commit peer changes from the resync task
 * MINOR: peers: sligthly adapt part processing the stopping signal
 * MINOR: peers: Add flags to report the peer state to the resync task
 * MINOR: peers: Add 2 peer flags about the peer learn status
 * MINOR: peers: Split resync process function to separate running/stopping states