allow users to free some compression contexts

[Didier Barvaux]

Requested by Mohammad Abyan Abdullah on question #276107

   /* free a context knowing its Context ID (CID) */
   bool rohc_comp_free_context(struct rohc_comp *const comp,
                               const rohc_cid_t cid);

   /* free all contexts of one profile whose last packet is older or equal than now - timeout */
   bool rohc_comp_free_old_contexts(struct rohc_comp *const comp,
                                    const rohc_profile_t profile_id,
                                    const struct rohc_ts timeout);

The 2nd function should allow users to free all contexts corresponding to RTP streams in timeout (according to their own definition of timeout).