This reverts commit 9265ea6a.

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.

A ICProxyAddr variable is usually named as "addr", so the attribute is
referred as "addr->addr", it's confusing and sometimes ambiguous.

So renamed the attribute to "sockaddr", the function
ic_proxy_extract_addr() is also renamed to ic_proxy_extract_sockaddr().

The GUC gp_interconnect_proxy_addresses is used to set the listener
addresses and ports of all the proxy bgworkers, only IP addresses were
supported previously, which is inconvenient to use.

Now we add the support for hostnames too, the IP addresses are also
supported.

Note that if a hostname is bound to a different IP at runtime, we must
reload the setting with the "gpstop -u" command.
Reviewed-by: NHubert Zhang <hzhang@pivotal.io>

Previously, when backends connect to a proxy, we need to setup
domain socket pipe and send HELLO message(recv ack message) in
a blocking and non-parallel way. This makes ICPROXY hard to introduce
check_for_interrupt during backend registeration.

By utilizing libuv loop, we could register backend in paralle. Note
that this is one of the step to replace all the ic_tcp backend logic
reused by ic_proxy currently. In future, we should use libuv to replace
all the backend logic, from registeration to send/recv data.
Co-authored-by: NNing Yu <nyu@pivotal.io>

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.