make block reading more efficient

...by using streams. This way we don't load all the blocks before parsing them.
We parse them as we go.

examples/blockreader.js

@@ -6,15 +6,22 @@ var BufferWriter = require('../lib/bufferwriter');
 //To use, pipe in a blk*****.dat file. e.g.:
 //cat blk00000.dat | node blockreader.js
 
-var bw = new BufferWriter();
+var head = null;
+var body = null;
 
-process.stdin.on('data', function(buf) {
-  bw.write(buf);
-});
-
-process.stdin.on('end', function(buf) {
-  var blocksbuf = bw.concat();
-  var br = new BufferReader(blocksbuf);
-  while (!br.eof())
-    console.log(JSON.stringify(Block().fromBufferReader(br).toJSON(), null, 2));
+process.stdin.on('readable', function() {
+  if (!head) {
+    head = process.stdin.read(8);
+    if (!head)
+      return;
+  }
+  var body = process.stdin.read(head.slice(4).readUInt32LE(0));
+  if (!body)
+    return;
+  var blockbuf = BufferWriter().write(head).write(body).concat();
+  var block = Block().fromBuffer(blockbuf);
+  console.log(block.toJSON());
+  head = null;
+  body = null;
+  process.stdin.unshift(process.stdin.read());
 });