Add the GMT offset in the rotated chunk path

The path of a rotated chunk is composed of the start and end timestamp
of the trace inside that chunk. In order to support distributed
environments, we now specify the GMT offset in this path as well. The
date is now formatted in ISO 8601. Here is an example:
~/lttng-traces/<session-name>/20180118T144610-0500-20180118T144611-0500-1

Chunk start: 2018-01-18 14:46:10
Chunk end: 2018-01-18 14:46:11
GMT offset: GMT-5 on both timestamps
Chunk ID: 1 (number of rotations that occured in this session so far)

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>