max_size should default to a sane value

It's been noticed that if no max_size param is passed to ep_engine, it defaults to an insanely large value. It should at least default to system memory size or something some percent below the default system memory size.

Admittedly, the scenarios where this could come up would be very unusual, but the behavior is very wrong if, for instance, something gets sent incorrectly as params to the system.