manifest: use a size 3 LRU cache to store parsed manifests Previously, the manifest cache would store the last manifest parsed. We could run into situations with operations like update where we would try parsing the manifest for a revision r1, then r2, then r1 again. This increases the cache size to 3 to avoid that bit of performance fragility.

util: add an LRU cache dict In certain cases we would like to have a cache of the last N results of a given computation, where N is small. This will be used in an upcoming patch to increase the size of the manifest cache from 1 to 3.

tests: remove last two check-code warnings about killdaemons Removes the last of the warnings in test-check-code-hg.t introduced in 667063b22a69.