Use default if there is no clock declaration

In absence of clock description in metadata, use a default clock
source which increments once per nanosecond.

Without this patch, the converter crashes when given old traces
that are lacking clock description block.

[ Edit by Mathieu Desnoyers: style cleanup, added clock description,
  added comments. ]

Signed-off-by: Juha Niskanen <juniskane@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>