cpp-common/bt2: rework `bt2::CommonIterator` (now `bt2::BorrowedObjectIterator`)

cpp-common/bt2: rework `bt2::CommonIterator` (now `bt2::BorrowedObjectIterator`)

We don't need all the STL stuff (and constraints) currently, therefore
this iterator class may be simpler, making operator*() call
ContainerT::operator[]() directly instead of keeping a temporary wrapper
object and having a conditional for each iteration increment.

Making operator->() return a proxy because the iterator instance has no
persistent wrapper instance.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I1bdb254a37533b8524450b22aa7f25a4d8025c2c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11236
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>