Increase memory available for javadoc generation

javadoc target is behaving unpredictably, sometimes fails, sometimes does not.
I strongly suspect that it just does not have enough memory available and it fails.
This is currently a blocker for releases. My empirical testing shows that more memory
we assign to javadoc generation, less probable it is it will fail to finish successfuly.

patch by Stefan Miklosovic for CASSANDRA-20868

Author: smiklosovic

build.xml

@@ -344,7 +344,7 @@
         windowtitle="${ant.project.name} API" classpathref="cassandra.classpath"
         bottom="Copyright © 2009- The Apache Software Foundation"
         useexternalfile="yes" encoding="UTF-8" failonerror="false"
-        maxmemory="256m" additionalparam="${jdk11-javadoc-exports}">
+        maxmemory="1024m" additionalparam="${jdk11-javadoc-exports}">
         <filesets/>
       </javadoc>
       <fail message="javadoc failed">