Make some progress for the cachedump command.

  I have got the requirement from my team that provide a approach to delete some of the keys from the memcached for several times, and they don't know the keys exactly(e.g. To delete keys with the same prefix). I think the feature to scan all the keys in the cached is useful, and the stats cachedump works not powerful enough.
  So, there is a little progress for the stats cachedump command, hoping it is helpful to others.


1. Support dump all keys in a slab without limitation of maximum memory 2M;
2. Support dump not-expired keys only, if you specify a negative limit number.