Several HConstant members are assignable

Several of the members in HConstants are not marked final.

Generate up-to-date list with command:
0.98+

for line in "`grep -e '^ *public' hbase-common/src/main/java/org/apache/hadoop/hbase/HConstants.java | grep -v final`"; do echo "${line}" | grep -o -e " [A-Z_][A-Z_]* "; done

0.94:

for line in "`grep -e '^ *public' src/main/java/org/apache/hadoop/hbase/HConstants.java | grep -v final`"; do echo "${line}" | grep -o -e " [A-Z_][A-Z_]* "; done

These mutable versions have been in many releases. We correctly don't assign to them in our codebase, but there's no way to know if anyone does downstream.

HConstants is IA.Public and IS.Stable, but I'm fairly certain this behavior was unintentional and there isn't a non-broken use for it.