BUG/MEDIUM: pollers: Use a global list for fd shared between threads.

BUG/MEDIUM: pollers: Use a global list for fd shared between threads.

With the old model, any fd shared by multiple threads, such as listeners
or dns sockets, would only be updated on one threads, so that could lead
to missed event, or spurious wakeups.
To avoid this, add a global list for fd that are shared, using the same
implementation as the fd cache, and only remove entries from this list
when every thread as updated its poller.

[wt: this will need to be backported to 1.8 but differently so this patch
 must not be backported as-is]