Clean-up: tests: bt2 plug-ins: modernize the plug-ins

Clean-up: tests: bt2 plug-ins: modernize the plug-ins

By virtue of their use of the C Babeltrace 2 APIs, the test plug-ins
perform a fair amount of manual resource management.

To make it possible to adopt a more modern C++ style in those plug-ins,
a number of helpers are introduced.

Introduce reference wrappers for the Babeltrace 2 interface:
  - value_ref: wraps a bt_value reference using std::unique_ptr
  - message_const_ref: wraps a constant message reference using a
    unique_ptr
  - message_iterator_ref: wraps a message iterator reference using a
    unique_ptr
  - event_class_const_ref: wraps a constant event class reference using
    a unique_ptr

A specialized random_access_container_wrapper is specialized to wrap
bt_value arrays of strings.

In doing so, it is possible to eliminate the use of gotos and manual
reference management on error paths. Some struct/classes are renamed to
eliminate ambiguities that arose over the refactoring.

The changes allow some simplifications of the code flow in places which
are applied directly.

Change-Id: I25c148d7970cb89add55a86f2c162973d3d27e4a
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>