conference: force RTP packet duration for PCMA and PCMU
MS A/V conferencing server requires P-time (duration of packet
payload in miliseconds) to be one of supported values. See [MS-RTP].
Accidentaly I had this values correct when testing commit
8675df9c0df44a7161a5ea4a1bc214943a39ba55, so it worked fine but failed
after I tried to run SIPE on different machine (seems 32 and 64 bit
GStreamer use different P-time defaults).
Also tried again to use SIREN with packetization times from the
document but still without success.