syncdaemon: give some refactoring to cascading code

- expiry check of foreign volinfo moved back to GLUSTERServer,
  so that under the hood we can removexattr the expired ones;
  a nice side-effect is that we can use the same dict layout
  for foreign and native volinfo (ie., foreign needs no
  timeout field)
- get_volinfo() is renamed to get_sys_volinfo() and most of the logic
  is stripped off of it (what remained there is the check
  against foreign master ambiguity)
- volinfo transition logic is cut out to an almost purely functional
  static method (only impurity is the exeption raised upon
  forbidden volinfo change)
- ping renamed to keep-alive, as something called "ping" is
  not supposed to have payload (yeah, keep-alive is a bit fishy
  on this front too, but could not come up with better...)

Signed-off-by: Csaba Henk <csaba@gluster.com>
Signed-off-by: Vijay Bellur <vijay@dev.gluster.com>

BUG: 2535 (gsync cascading)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=2535

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