Cache sizing is critical for performance.  This change guesses at a
reasonable setting for the WiredTiger cache size at runtime.  This cache
is shared across all vnodes regarless 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.
Merge pull request #5 from basho-labs/gsb-cache-size-est

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