use bulk ByteBuffer.put() method in getWritableDataPointer()

[dwrensha]

Similar to #143, we should update these other places:

capnproto-java/runtime/src/main/java/org/capnproto/WireHelpers.java

Lines 876 to 879 in 08009f3

	// TODO is there a way to do this with bulk methods?
	for (int i = 0; i < builder.size; ++i) {
	builder.buffer.put(builder.offset + i, defaultBuffer.get(defaultOffset * 8 + i));
	}

capnproto-java/runtime/src/main/java/org/capnproto/WireHelpers.java

Lines 805 to 808 in 08009f3

	// TODO is there a way to do this with bulk methods?
	for (int i = 0; i < builder.size; ++i) {
	builder.buffer.put(builder.offset + i, defaultBuffer.get(defaultOffset * 8 + i));
	}

[dwrensha]

done in b2f7242