The peer addresses are specified with the GUC
gp_interconnect_proxy_addresses, it can be reloaded on SIGHUP, we used
to only care about the newly added ones, however it is also possible for
the user to modify them, or even remove some of them.

So now we add the logic to classify the addresses after parsing the GUC,
we can tell whether an address is added, removed, or modified.

The handling of the classified addresses will be done in the next
commit.

A typical mistake on allocating typed memory is as below:

    int64 *ptr = malloc(sizeof(int32));

To prevent this, now we make ic_proxy_new() a typed allocator, it always
return a pointer of the specified type, for example:

    int64 *p1 = ic_proxy_new(int64); /* good */
    int64 *p2 = ic_proxy_new(int32); /* bad, gcc will raise a warning */
Reviewed-by: NHubert Zhang <hzhang@pivotal.io>

We used to store them under /tmp/, we include the postmaster port number
in the file name in the hope that two clusters will not conflict with
each other on this file.

However the conflict still happen in the test src/bin/pg_basebackup.
And it can also happen if a second cluster is missed configured by
accident.  So to make things safe we also include the postmaster pid in
the domain socket path, there is no chance for two postmasters to share
the same pids.
Reviewed-by: NPaul Guo <pguo@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.