lttng project: update from 2.3.0 to 2.4.1

(And update liburcu to 0.8.4 according to release notes for lttng 2.4.x.)

In addition to new features and bug fixes, version 2.4.x is needed to build
against Linux 3.12 (our new stable kernel).
babeltrace: update from 1.1.1 to 1.2.1