See: SERVER-23624, SERVER-23658, SERVER-23624

The minimum floor for wiredTigerCacheSizeGB has been lowered from 1GB to 256MB to better support low memory machines. Also, the default setting has been lowered from 60% to 50% of available memory (I believe the documentation was never updated to reflect the 60% change made in 3.2, so this may be a no-op).

To better support fine grained control of the cache size, it is also now possible to specify the setting as a float, for example --wiredTigerCacheSizeGB 2.4, etc.

The maximum cache size remains 10TB.

These settings are the same for both the WiredTiger and the inMemory storage engine. This change is onlyl in effect for current master which will become 3.4.0