Parse CTF indexes

If a trace file has an associated index (same filename and .idx suffix
in the "index" folder), we open it and use it instead of generating
the index at open.
The index folder is not mandatory and can be destroyed without any
problem.

To use with LTTng, make sure you have lttng-tools commit :
50adc26400482c07210afcda8ef1d3322f75871d
Mon Nov 25 15:32:38 2013 -0500
Rename LTTng index in CTF index

Versions between commit 309167d2a6f59d0c8cbf64eb23ba912cdea76a34 and
this commit have a different index format that will never be merged in
Babeltrace, so if you have traces recorded between this period of
time, just remove the index directory.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>