Bump up default BDB cache- and allowed mmap size considerably

Bump up default BDB cache- and allowed mmap size considerably
- The former defaults come from nearly a decade ago, things have
  changed a bit since then... Even BDB's own default cache size is
  much larger (8MB) now than what we've been setting.
- Using 64MB cache improves cache hit rate (and performance) massively.
  Last-gasp default to roughly BDB's own current defaults, higher
  settings in the default macro setup.