api: better segregate client and server code

api: better segregate client and server code

 - move memory and server specific vl_msg_api_handler_with_vm_node()
   to memory server code only
 - keep api_global_main static

Apart from being cleaner, this also helps avoiding symbols conflict
when both client and server libs are loaded in the same process, as is
done by the prom plugin.
Those symbols conflict confuse ASan and can be nasty to debug.

Type: improvement

Change-Id: Iaf58596cc753ad8d3fedd8d65c4bf480ac129c2c
Signed-off-by: Benoît Ganne <bganne@cisco.com>