[Fiber] Make isRunnable() perform a relaxed load and add documentation

It is safe to perform a relaxed load in isRunnable(), since the
information is potentially outdated immediately after it was looked

Also, since Fiber::isRunnable() is a virtual method, keeping its
definition in the header has no real advantage, hence we move it out
of the header.
44 jobs for abstract-fiber in 14 minutes and 43 seconds (queued for 2 minutes and 8 seconds)