While we work on a replacement for the lookaside score (
WT-5499), to unblock us from compiling a patch of durable history with MongoDB, add lookaside score statistic and always set it to zero.
Also, add cache_overflow config option back as a synonym to history_store for setting a maximum size for the history store file. This is also needed for MongoDB to compile for now, till we make a server change to switch to using history_store.
Both of these will eventually be removed from develop, after a server change switches to new statistic and configuration.