Correct the HyperLogLog stale cache flag to prevent unnecessary computations.

Set the MSB as documented.
Cluster test: proper initialization at unit startup.
Cluster test: failure detection initial tests.
Cluster test: better failure detection test and framework improvements.
Merge pull request #1762 from trink/unstable

Correct the HyperLogLog stale cache flag to prevent unnecessary computat...
HyperLogLog regression test for issue #1762.