common: replace container_of with a C++ safe implementation

As more code moves to a more idiomatic C++ style, structures like
typically end up becoming classes that use different access controls,
virtual functions, etc. This, in turn, makes them adopt a non standard
layout and causes GCC and clang to emit the following warning when
container_of is used:

error: 'offsetof' within non-standard-layout type 'foo' is conditionally-supported [-Werror=invalid-offsetof]

This new implementation of container_of makes use of a pointer to a data
member to find the parent's address.

The use of ptr_to_member against the null dummy_parent makes me uneasy
as it seems equivalent to performing arithmetic on a null pointer, which
I understand is undefined behavior (C++11 Standard 5.7.5).

However, Boost.Instrusive uses an approach that seems roughly equivalent
to lttng::utils::container_of() [1].

It seems like a reasonable compromise that works on all mainstream
compilers.

[1] https://github.com/boostorg/intrusive/blob/3c5c8cec3f0356a028a4b56ba6cac2256340dab1/include/boost/intrusive/detail/parent_from_member.hpp#L92

Change-Id: Ia6287e1648bce85dfe6de936f17ec5df46ea648d
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>