Wrong use of cache tags in FieldInfo class

The class does multiple cache sets with array('field_info' => TRUE), but that should just be array('field_info'). Somehow this causes those cache entries to never be valid, so there are no cache hits and the information is rebuild every time it is requested.

Discovered in #1810178: field_info_extra_fields() is not language-aware, may return wrong values.

We have written multiple tests already recently that use a new memory counter backend to check the set/get calls within a test. We could do the same here. See #1969814: system_list() does a cache call on every function call, for example. Although I'm not sure if the memory backend behaves the same way as the database... but that would be a bug there, in either of those backends.