Firestore: 'Query.where' needs to raise for sentinels / transforms

[tseaver]

The following conformance tests assert that attempting to use DELETE_FIELD, SERVER_TIMESTAMP, ArrayRemove, or ArrayUnion as the value of a Query.where must raise an exception, which the current implementation does not do:

• query-arrayremove-where.textproto
• query-arrayunion-where.textproto
• query-del-where.textproto
• query-st-where.textproto