CA-104483: rrdd: ensure plugins' checksums change
CA-104483: rrdd: ensure plugins' checksums change

Previously, rrdd only checked whether the plugin's checksum is different
from the first one it saw for that plugin. This had the effect that, if
the checksum changed at least once since rrdd was started, the data was
always deemed to be fresh. The logic was flawed: Hashtbl.replace was only
called when the entry wasn't previously in the hashtable.

Instead, this patch checks the current checksum against the most recently
seen one. The effect of this is that data is only deemed fresh if it has
been updated recently.

Imported from xen-api.git a4a3e983bb9cff75a1fd504c677eb8db28ad9979

Signed-off-by: Jonathan Davies <d3af50858a27f644d377a39d1fbd399608163688@citrix.com>
Imported-by: John Else <37b74501daa612e48fea94f9846ed4ddeb6a8f07@citrix.com>

johnelse/xcp-rrdd,koushikcgit/xcp-rrdd,koushikcgit/xcp-rrdd,koushikcgit/xcp-rrdd,johnelse/xcp-rrdd,djs55/xcp-rrdd,simonjbeaumont/xcp-rrdd,djs55/xcp-rrdd,simonjbeaumont/xcp-rrdd