Fix Windows mmap sizing.

## Overview

This commit fixes an issue on Windows where the database was doubling when it was re-opened. This occurred because Windows has to truncate the file to the mmap size and the mmap resizing code was doubling the size whenever the DB size was at the next threshold. This has been changed so that the DB size will double only when the DB size is above the next threshold.

Fixes #216

/cc @rusco @natefinch