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>

Resource group used to access resGroupSlot in SessionState without
lock. This is correct when session only access resGroupSlot by itself.
But as we introduced runaway feature, we need to traverse the current
session array to find the top consumer session when redzone is reached.
This requires:
1. runaway detector should hold shared resgroup lock to avoid resGroupSlot
is detached from a session concurrently when redzone is reached.
2. normal session should hold exclusive lock when modifying resGroupSlot
in SessionState.

Also fix a compile warning.
Reviewed-by: NNing Yu <nyu@pivotal.io>

Proxy bgworker will become orphan process after postmaster is dead
due to the lack of checking pipe postmaster_alive_fds[POSTMASTER_FD_WATCH].
Epoll this pipe inside proxy bgworker main loop as well.
Reviewed-by: NNing Yu <nyu@pivotal.io>

Fixed the bug that the SIGHUP handler was installed for SIGINT by
mistake, so the ic-proxy bgworkers would die on SIGHUP.

By correcting the signal name, now we could let the ic-proxy bgworkers
reload the postgresql.conf by executing "gpstop -u".
Reviewed-by: NHubert Zhang <hzhang@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.