Seeding random generator each time limits number of generated sequence
to 31-bits, and caused collision.
Use global random generator seeded by crypto grade random.
Use crypto/rand for cryptographic values,
and math/rand for unique identifier.
- Use UUIDv4 for mDNS name
- Use crypto/rand for ICE pwd and user fragment
- Use properly seeded math/rand for UDP port,
tie breaker and candidate ID
