Handle transient local messages in snapshot mode when the buffer is full

[doisyg]

Draft to discuss ways of solving #1886

Naive solution which throws away the first message that is not transient local when the buffer is full, as long as the number of messages at the beginning of the buffer accounts for less than 10% of the total number of messages in the buffer.

That will not solve completely the /tf_static use case if other transient local messages are recorded and periodically published: at some point there will be more than 10% of transient local messages in the buffer, and the first ones (i.e /tf_static) will be removed.

Interested in debating and suggestions on how to solve this properly :

• Different buffers for each transient local topic ? But then how to control their max size ?
• A buffer dedicated to /tf_static? But then loss of generality.
• Something else ?

[doisyg]

(PR non functional for Rolling, I was originally targeting Iron, will fix)