Improve performance of getLinkedOps

Running `getOps` to fetch a large number of operations is not very
performant.

Anecdotally, performing this operation on a local development machine
with a document comprising of ~200,000 operations takes ~20s.

The largest slow-down appears to come from the `getLinkedOps` method,
and in particular the use of `Array.unshift`.

This change "inverts" the logic of the `getLinkedOps` function. Rather
than building up an array from scratch, instead we create a shallow copy
of the `ops` array using `slice`, and remove any operations we don't
want using `splice`.

The anecdotal operation time using this modified function is now ~2s,
which is an improvement of an order of magnitude. The performance when
fetching a small number of operations is similar.

Author: Alec Gibson

index.js

@@ -526,15 +526,21 @@ function getLatestDeleteOp(ops) {
 }
 
 function getLinkedOps(ops, to, link) {
-  var linkedOps = []
+  var linkedOps = ops.slice(0);
+
   for (var i = ops.length; i-- && link;) {
     var op = ops[i];
-    if (link.equals ? !link.equals(op._id) : link !== op._id) continue;
+
+    if (link.equals ? !link.equals(op._id) : link !== op._id) {
+      linkedOps.splice(i, 1);
+    }
+
     link = op.o;
     if (to == null || op.v < to) {
       delete op._id;
       delete op.o;
-      linkedOps.unshift(op);
+    } else {
+      linkedOps.splice(i, 1);
     }
   }
   return linkedOps;