Cleaned up reconfiguration sequence.

mainReconfigure() used to close and then open various sockets. Since
comm_close is now asynchronous, one cannot close and open in the same
function. Split mainReconfigure into mainReconfigureStart (that starts
the closing process for all relevant sockets) and mainReconfigureFinish
that opens the new sockets.

serverConnectionsClose is only used by main.cc and, hence, can be
static.

Polished comments and added an XXX comment on why SquidShutdown is
broken.

Also removed commCheckHalfClosed event scheduling. A separate cleanup
patch removes the associated half-closed monitoring loop.