Record Fetch Collapse: Collapse multiple fetch request into one single request.

Record Fetch Collapse: Collapse multiple fetch request into one single request.

When multiple clients fetch the same record concurrently, send only one single
fetch across the network and deferr all other fetches locally.
This improves performance for hot records and reduces cpu load on ctdb.

(This used to be ctdb commit 82d6946ad8b3348e8b9d3d971f24925ade02d1be)