MINOR: threads/fd: Use a bitfield to know if there are FDs for a thread in the FD...

MINOR: threads/fd: Use a bitfield to know if there are FDs for a thread in the FD cache

A bitfield has been added to know if there are some FDs processable by a
specific thread in the FD cache. When a FD is inserted in the FD cache, the bits
corresponding to its thread_mask are set. On each thread, the bitfield is
updated when the FD cache is processed. If there is no FD processed, the thread
is removed from the bitfield by unsetting its tid_bit.

Note that this bitfield is updated but not checked in
fd_process_cached_events. So, when this function is called, the FDs cache is
always processed.

[wt: should be backported to 1.8 as it will help fix a design limitation]