Add config option `heap_per_environment_ratio`

[jtnelson]

This config will help the Database size its internal stores and caches. Value should be a number. IF it is above 1, it is considered a percent. If it is a decimal it will be treated as the multiple of the configured heap size. Make clear in documentation that an environment may use more heap than the configured target (if necessary), but this is provided to appropriately size things based on the anticipated number of environments.

Default value should be 1 or 100 (100 %)