MEDIUM: fd: Add the tune.fd.tables option

MEDIUM: fd: Add the tune.fd.tables option

Add a new experimental option, tune.fd.tables, that can be set to
either "shared" (the default) or "per-thread-group", to enable
per-thread group file descriptor tables.
This can give a nice performance boost when running with a lot of
threads, as we observe a lot of contention on the file descriptor table
lock in the kernel.