Fix b2bua

• Add config options to force SAVP and AVPF
  • Write unit tests
• Forward DTMF
  • Write unit tests
  • Fix SIP INFO fallback
    • Write unit tests
• Fix Contact matching by callid in registrar
  • Write unit tests
  • Verify that liblinphone_tester Register / Registration with custom contact is indeed fixed
• Fix call slots never deallocated. (I now see 62 free slots out of 100 in the client's test server, and no traffic on the B2BUA)

Child: linphone-sdk!3131 (merged)