Added caching

[wouterj]

Port of symfony-cmf/routing-auto-bundle#78

[dantleech]

For me this is more of a desirable feature, not a "blocker" - it needs a rebase ..

[wouterj]

It's less than a blocker, but more than a "nice to have" imo :)

[dantleech]

I find this line extremely difficuly to understand, I would prefer something more structured like the following:

(infact I only understood what it did after writing the following code)

  $resolvedMetadata = null;

  // if there is cache
  if (null !== $this->cache) {
      $metadata = $this-loadClassMetadataFromCache($reflection);

      if ($metadata->isFresh) {
          $resolvedMetadata = $this->getCachedMetadata($reflection);
      } else {
          $resolvedMetadata = $this->loadMetadataForClass($class, $metadata, $addedClasses);

          if (null === $resolvedMetadata) {
              $resolvedMetadata = $this->resolveMetadata($class, null, $addedClasses);
          }

          if (null === $resolvedMetadata) {
              return null; // throw exception??
          }

          $this->putClassMetadataInCache($resolvedMetadata);
      }

      $this->resolvedMetadata[$class] = $resolvedMetadata;
  }

  return $this->resolvedMetadata[$class];

[dantleech]

Why close this? Would we do this in a different way now?

[wouterj]

This PR has been open for very long and I can no longer follow the logic in here. Maybe I'll write caching again in some other PR.