Create empty set if it doesn't exist
Improve proxy support

Re-write proxy support and refresh support to enable more diverse
deployment scenarios.

Proxy packets now have an area ID and TTL field to help proxies
determine where packets come from and where they should, and
should not go to.

VXFLD server lists (vxfld_refresh_servers, vxfld_proxy_servers,
vxfld_listen_address) now support optionally including a port. If
the port is not specified then the value of the vxfld_port option
is used. All code will now send packets to the destination port
specified in the server definition. Additionally it will listen
on all IPs and ports specified.

New config options for vxsnd:
proxy_local_only:
    When set to True a proxy will only forward packets from the
local area to other areas. This help prevents config errors causing
proxy loops. Default value is True.

enable_vxlan_list:
    When set to True vxsnd will listen for vxlan packets on the
vxlan_listen_address and vxlan_port. This option allows proxies
to only listen for proxy requests and not VXLAN requests. Default
value is True.

proxy_id:
    The IP address a proxy should use to identify it's self in
a proxy packet. This defaults to the first IP address listed in
the vxfld_list_address config option.

refresh_proxy_servers:
    When set to True vxsnd will also send refresh messages to
proxy servers defined in vxfld_proxy_servers as well as
vxfld_refresh_servers.
Add support for fdb sync on startup

Allow vxsnd to request an fdb sync from proxies or refresh servers
on startup.

If configured, on start vxsnd will select a configurable number
of proxies and refresh servers to request an fdb sync. The vxsnd
process will send a Refresh packet a response_type (new field)
of "all".

When vxsnd recieved a Refresh packet with a response_type of
"all" it will send a Refresh packet for each VNI back. The
fdb is sent one VNI per packet to prevent packet overflow.

New config options
sync_fdb:
    If true, vxsnd will request an fdb resync from refresh
servers. Default value of true.

sync_targets:
    Number of refresh servers to request an fdb sync from. If
sync_targets is less then the total number of refresh servers then
vxsnd will randomly select sync_targets number of servers from the
available list of refresh servers.

sync_from_proxy:
    If true, vxsnd will also request a resync from proxy servers.
proxy servers always take precedence over refresh servers so it will
first request a refresh from up to sync_targets proxy servers. If
there are less proxy servers then sync_targets it will then randomly
select from the list of refresh servers until sync_targets. Default
value of false.