rtpjitterbuffer: already_lost() function assumed packet are lost too soon

The already_lost() function, lookup for the a seqnum within a range of a LOST event (for the name num > 1, so that means equidistant mode). This is slightly redundant with the previous find_timer() call (adding a second o(n) lookup), but the main issue is that this function assumes that whenever a lost timer is found in the timers array, that packet is definitely lost. A packet is only difinitly lost when the lost timer is executed by the timer thread. A test should be written to demonstrate that some "just before late" packet will be dropped by the jitterbuffer in equidistant mode if a gap containing this packet was detected before. This code was introduced by 8f7962e1 which seems to fix a valid issue, but with side effect.

cc @hgr

p.s. This is slightly related to #608 (closed) since the timer compression implementation is incomplete