Change wfq usages for wfcq

This removes the deprecated warnings when building lttng-tools. We can
now build with -Werror, woohoo!

This makes lttng-tools depends on userspace-rcu version 0.8.0 and above.
The configure.ac and README files have been updated for this.

Verified by running make check.

Signed-off-by: David Goulet <dgoulet@efficios.com>