Rename Snapshot middleware "historical" flag to "byVersion"

Calling a snapshot "historical" might lead people to assume that it's
not the current version, which may not be the case (for example, if you
fetch the current version by its ID with `fetchSnapshot` or, pass in
a version of `null`).

This change updates the snapshot middleware to use the more descriptive
`byVersion`, which indicates that the snapshot was fetched by version.

Author: Alec Gibson

lib/backend.js

@@ -80,8 +80,8 @@ Backend.prototype.MIDDLEWARE_ACTIONS = {
 Backend.prototype.SNAPSHOT_TYPES = {
   // The current snapshot is being fetched (eg through backend.fetch)
   current: 'current',
-  // An historical snapshot is being fetched (eg through backend.fetchSnapshot)
-  historical: 'historical'
+  // A specific snapshot is being fetched by version (eg through backend.fetchSnapshot)
+  byVersion: 'byVersion'
 };
 
 Backend.prototype._shimDocAction = function() {
@@ -615,7 +615,7 @@ Backend.prototype.fetchSnapshot = function(agent, index, id, version, callback)
     if (error) return callback(error);
     var snapshotProjection = backend._getSnapshotProjection(backend.db, projection);
     var snapshots = [snapshot];
-    var snapshotType = backend.SNAPSHOT_TYPES.historical;
+    var snapshotType = backend.SNAPSHOT_TYPES.byVersion;
     backend._sanitizeSnapshots(agent, snapshotProjection, collection, snapshots, snapshotType, function (error) {
       if (error) return callback(error);
       backend.emit('timing', 'fetchSnapshot', Date.now() - start, request);

test/client/snapshot-request.js

@@ -228,7 +228,7 @@ describe('SnapshotRequest', function () {
         backend.use(backend.MIDDLEWARE_ACTIONS.readSnapshots,
           function (request) {
             expect(request.snapshots[0]).to.eql(v3);
-            expect(request.snapshotType).to.be(backend.SNAPSHOT_TYPES.historical);
+            expect(request.snapshotType).to.be(backend.SNAPSHOT_TYPES.byVersion);
             done();
           }
         );