On-disk multiple tracefiles circular buffer

This patch introduces the tracefile_size and tracefile_count parameters
to the enable-channel command and the API.

This allows to split a stream into multiple tracefiles and limit the
amount of trace data to keep on disk. The tracefiles are readable
independently or with the others as long as the metadata file is
present.

For now only local traces are handled, relayd modifications coming soon.

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