sessiond: cmd_rotate_session: introduce command argument

Add a command type argument to cmd_rotate_session().

Change the internal session_close_trace_chunk() API to introduce the
LTTNG_TRACE_CHUNK_COMMAND_TYPE_NO_OPERATION command rather than rely on
a NULL pointer, which facilitates passing of command argument across
cmd_rotate_session.

This introduces the LTTNG_TRACE_CHUNK_COMMAND_TYPE_DELETE command,
which is required for the clear feature. Its implementation will
follow in a separate commit.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ic1d8cbab1f98b93f1f6d27a2c6624ac2780932e2
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>