obscache: stop definition of the empty key

obshashrange: keep value fetched from sql in memory This will reduce the number of roundtrip to the data base we need.

obshashrange: stop inheriting from 'dict' A simple dictionary attribute seems more simpler.

merge with stable

obshashrange: to not overwrite the list with the set We reusing the list variable lower in the code.

obshashrange: properly break out of the two loops Otherwise we would continue to iterate even after we reseted the cache.

obshashrange: fix reset conditional If the revision is in "revs" we should not reset the cache. The old code was wrong (Thanks goes to new tests for catching this).

cache: ensure we warm stablerange cache before the obshashrange cache I've been seeing traceback that seems to be happening because of issues in this area.

evolve: fixing obscache invalidation This was missing a call to the parent's destroyed(), such that any transaction after stripping some nodes would result in a crash (by attempting to read nodes which were stripped).

obshashrange: warm the cache at the end of each transaction This will help having warmed cache for read only client. The warming is still imperfect in case of markers that trigger a reset, but we are in a better place than what we used to be.