Pipeline requests in get_multi

Instead of doing lot's of small writes do 1 larger write. This also
considerably cuts down on locking already locked servers.
Keep existing behaviour when no server is found
Debug message instead of warning

Closer to old behaviour
Make get_multi go through chokepoint method
Merge branch 'faster_get_multi_rebased' of git://github.com/xaop/dalli into xaop-faster_get_multi_rebased
Update changes, version