api: remove transport specific code from handlers

api: remove transport specific code from handlers

This does not update api client code. In other words, if the client
assumes the transport is shmem based, this patch does not change that.
Furthermore, code that checks queue size, for tail dropping, is not
updated.

Done for the following apis:
Plugins
- acl
- gtpu
- memif
- nat
- pppoe
VNET
- bfd
- bier
- tapv2
- vhost user
- dhcp
- flow
- geneve
- ip
- punt
- ipsec/ipsec-gre
- l2
- l2tp
- lisp-cp/one-cp
- lisp-gpe
- map
- mpls
- policer
- session
- span
- udp
- tap
- vxlan/vxlan-gpe
- interface
VPP
- api/api.c
OAM
- oam_api.c
Stats
- stats.c

Change-Id: I0e33ecefb2bdab0295698c0add948068a5a83345
Signed-off-by: Florin Coras <fcoras@cisco.com>