The user can adjust the ic-proxy peer addresses at runtime and reload by
sending SIGHUP, if an address is modified or removed, the corresponding
peer connection must be closed or reestablished.  The same to the peer
listener, if the listener port is changed, then must re-setup the
listener.

ic-proxy is developed with libuv, the minimal supported libuv version is
1.18.0. But in commit 608514, we introduce new API in libuv 1.19, which
break compatibility on os like Ubuntu 18.04 whose default libuv version
is 1.18.

We should keep our code base align with libuv 1.18, and replace the new
API in libuv 1.19. The API change is mainly about how to access data field
in uv handle and uv loop. The new API uses function interface like
`uv_handle_set_data` and `uv_handle_get_data` to access data filed, while
the old API in 1.18 access the data filed directly. Note that in the latest
libuv version 1.38.2, the old API and new API are both supported. And libuv
is stable enough to support the old API for a long time.

The interconnect proxy mode, a.k.a. ic-proxy, is a new interconnect
mode, all the backends communicate via a proxy bgworker, all the
backends on the same segment share the same proxy bgworker, so every two
segments only need one network connection between them, which reduces
the network flows as well the ports.

To enable the proxy mode we need to first configure the guc
gp_interconnect_proxy_addresses, for example:

    gpconfig \
      -c gp_interconnect_proxy_addresses \
      -v "'1:-1:10.0.0.1:2000,2:0:10.0.0.2:2001,3:1:10.0.0.3:2002'" \
      --skipvalidation

Then restart to take effect.