Upgrade from 4.0.7 to 4.0.8.

<ChangeLog>

Upgrade urgency CRITICAL ONLY for Redis Cluster users. Otherwise no reason
to upgrade at all.

Redis 4.0.8 fixes a single critical bug in the radix tree data structure
used for Redis Cluster keys slot tracking. The problem was actually fixed
10 months ago into unstable, but it was fixed in a commit related to Streams
so it was never backported (for error) into the 4.0 branch.

The problem will crash Redis Cluster instances during deletions, but it is
very hard to trigger: only when the node removed is in the edge of a memory
mapped area there are the conditions to create an issue, because otherwise
the code just accesses an out of range word in read-only way in an allocated
structure: this is almost always harmless.

</ChangeLog>