Estimate a reasonable cache size if one isn't provided in app/env config for wterl.

This change guesses at a reasonable setting for the WiredTiger cache size at runtime.  This cache is shared across all vnodes regardless of how many are active at any given time.  The algorithm is: max(1GB, 1/3 (RAM - Beam RSS size)). We don't enable direct_io on purpose and data will be double buffered in WiredTiger's cache and the filesystem buffer cache.  This turns out to be faster than direct I/O despite wasting a bit of RAM.