Add library load/unload tracking events

Track library load and unload, with the following new events:

lttng_ust_lib:load
lttng_ust_lib:build_id
lttng_ust_lib:debug_link
lttng_ust_lib:unload

This takes care of correctly tracing the mapping of direct dependencies
of dlopen'd libraries, which was not appropriately done by tracing just
dlopen events.

Fixes: #1035
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>