Switch back to former, much smaller BDB memory pool size (RhBug:752897)

Switch back to former, much smaller BDB memory pool size (RhBug:752897)

- A larger cache is beneficial in various scenarios, but triggers
  horrible worst-case performance under memory pressure (or so my
  current theory goes, there might be other factors too). The
  worst-case degration is orders of magnitude bigger than the best-case
  improvements from the larger cache and for many use-cases doesn't
  make a whole lot difference.  We could/should tune the cache with
  priorizing indexes and all, and perhaps dynamically select the
  cache size but for now, the 1Mb cache size is known to "just work".