RFR: 8246044: ZGC: Rename ZDirector's max_capacity to soft_max_capacity

[Per Liden]

After JDK-8222182, ZDirector started to use the "soft max capacity" to 
guide its heuristics. However, a number of local variables are still 
called max_capacity. I propose we rename them soft_max_capacity for clarity.

Bug: https://bugs.openjdk.java.net/browse/JDK-8246044
Webrev: http://cr.openjdk.java.net/~pliden/8246044/webrev.0

/Per