This function will pause an instance of postgres which is part of a GPDB
1) pause the postmaster (this prevents new connections from being made)
2) get list of processes that are descendent from postmaster process
3) pause all descendent processes and ignore any failures (a process may have died betwen getting pid list and doing the pauses)
4) again, get list of processes that are descendent from postmaster process
5) pause all descendent processes and ignore any failures (do not ignore errors, errors are failures, no pids can die between being paused the first time and getting the pid list)
"""
datadir=db.getSegmentDataDirectory()
content=db.getSegmentContentId()
postmasterPID=read_postmaster_pidfile(datadir)
ifpostmasterPID==0:
raiseException,'print "could not locate postmasterPID during pause'