* Fix constantly increasing memory in std::list

When someone is constantly publishing with the same tf2 timestamp
(application error, I know), the storage_ of the tf2::TimeCache grows
unbounded causing system-wide memory leaks. In a ROS system, each tf2
Listener will spawn one of these TimeCache objects and thus, all ROS
nodes that have any sort of tf2 listener will slowly start allocating
more and more memory

And also fixed the situation where two different tf2 needs to be
introduced at the same timestamp.

Since the tests were failing I realized that I had to dive a bit deeper
into the simple solution. And extend a bit the TransformStorage clase to
be comparable, now instead of just looking to the timestamps, the
implementation avoids inserting duplicates in the buffer.

If for some reason, someone is publishing a different Transfrom
(xyz,rpy) at the same timestamp. Well, that case is not being captured
now, but I'd say that's another problem

Signed-off-by: Ignacio Vizzo <ignaciovizzo@gmail.com>
(cherry picked from commit 1621942)