We've recently introduced IPv{4,6}Adapter construct classes and can
switch to this instead of using the old imperative encoder/decoder
functions {enc,dec}_addr_tlv().
Aside from code cleanup, this also means we now support the IPv6 address
type in EF.PCSCF.
Change-Id: I4d01ccfe473a8a80fbee33fdcbd8a19b39da85ac