enable spa to support fls

Hi Dave,

Please review the pull request. One thing I wasn't entirely happy with the patch is the getRequestId() method in FieldLevelSecurityFilter interface. It is used to work with the cache key within SoqlBuilder for expanded fields. With user based fls coming into play, I feel this cache is too embedded in the builder and becomes out of reach for maintenance. What do you think?

Thanks,
Alfred