chore: Added more checks on filtering

* Query checks whether property filter is sorted first
* Filter warns if filtered property is unindexed
chore: Added KEY_PROPERTTY on entity

For filtering by key property
keyProperty now has const constructor
Querying on unindexed property raises exception
bug: key property now indexed

So it will now not raise an exception when used in a filter
Merge branch 'master' of https://github.com/ovangle/googleclouddatastore