[GH-552] Added new query hints: HINT_NO_STORE_IDENTITY_MAP, HINT_NO_LOAD_ASSOCIATED_SUBTYPES

This issue is created automatically through a Github pull request on behalf of Koc:

Url: https://github.com/doctrine/doctrine2/pull/552

Message:

1. `HINT_NO_STORE_IDENTITY_MAP` usable for situations when we have article with big text fields and use partial loading for list articles. When we try load article by id from loaded list on same request we got partial entity. `HINT_NO_STORE_IDENTITY_MAP` allow avoid this.

2. `HINT_NO_LOAD_ASSOCIATED_SUBTYPES` usable when we have entity which has `one-to-one` or `many-to-one` association to abstract entity with subtypes.