[hotfix] [gelly] Reduce maximum number of generator blocks

The default maximum akka transfer size is 10 MB. This commit reduces the
number of generator blocks from 2^20 to 2^15 which removes the limit on
graph size. The original limit of one millions blocks was intended to
future-proof scalability.

This is a temporary fix as graph generation will be reworked in FLINK-3997.

flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/generator/random/AbstractGeneratorFactory.java

@@ -35,7 +35,7 @@ implements RandomGenerableFactory<T> {
 	// A large computation will run in parallel but blocks are generated on
 	// and distributed from a single node. This limit should be greater
 	// than the maximum expected parallelism.
-	public static final int MAXIMUM_BLOCK_COUNT = 1 << 20;
+	public static final int MAXIMUM_BLOCK_COUNT = 1 << 15;
 
 	// This should be sufficiently large relative to the cost of instantiating
 	// and initializing the random generator and sufficiently small relative to