BUG/MINOR: fd: Don't clear the update_mask in fd_insert.

BUG/MINOR: fd: Don't clear the update_mask in fd_insert.

Clearing the update_mask bit in fd_insert may lead to duplicate insertion
of fd in fd_updt, that could lead to a write past the end of the array.
Instead, make sure the update_mask bit is cleared by the pollers no matter
what.

This should be backported to 1.8.
[wt: warning: 1.8 doesn't have the lockless fdcache changes and will
 require some careful changes in the pollers]