RFR (JAXP): 8061686: Size limits in BufferAllocator should have been final

[huizhe wang]

Hi,

The three fields in com.sun.xml.internal.stream.util.BufferAllocator are 
used internally only, should have been private and final.

https://bugs.openjdk.java.net/browse/JDK-8061686
http://cr.openjdk.java.net/~joehw/jdk9/8061686/webrev/

Tests passed.

Thanks,
Joe