relayd fix: trace chunk is reclaimed before close command

The relay daemon control protocol defines a trace chunk close command
which allows the tracer to express the 'end' time bound of a trace
chunk.

However, in the event of a session rotation, the last reference to
such a trace chunk can be released before the close command is
received. This prevents the trace chunk from being renamed and moved
at the completion of a rotation.

A reference to a 'pending closure' trace chunk is kept as part of the
relay session object until its 'close' command is received.

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