Description:
------------
I noticed that libmemcached implemented a function to receive all keys of the servers named memcached_dump.
-> http://docs.tangent.org/libmemcached/memcached_dump.html

This would be very useful and could be implemented similar to APCIterator.

Reproduce code:
---------------
$flags = Memcached::DUMP_NONE // = 0
       | Memcached::DUMP_VALUE // receive data
       | Memcached::DUMP_MTIME // receive mtime
// ... possible flags

$rsSet = $memcached->dump($flags);
var_dump($rsSet);
foreach ($rsSet as $key => $rs) {
    var_dump($key;
    var_dump($rs);
}

Expected result:
----------------
object(MemcachedResultSet)#1 (0) {
}
string(7) "testKey"
array(2) {
  ["value"]=>
  string(8) "testData"
  ["mtime"]=>
  int(1268522244)
}
// ...

Actual result:
--------------
-