A persistent property store

WHAT
A persistent property store usable as a support facility for any metadata augmentation feature.

WHY
When adding new meta-data oriented features, we usually need to persist information linking the feature data and the HiveMetaStore objects it applies to. Any information related to a database, a table or the cluster - like statistics for example or any operational data state or data (think rolling backup) - fall in this use-case.
Typically, accommodating such a feature requires modifying the Metastore database schema by adding or altering a table. It also usually implies modifying the thrift APIs to expose such meta-data to consumers.
The proposed feature wants to solve the persistence and query/transport for these types of use-cases by exposing a 'key/(meta)value' store exposed as a property system.

HOW
A property-value model is the simple and generic exposed API.
To provision for several usage scenarios, the model entry point is a 'namespace' that qualifies the feature-component property manager. For example, 'stats' could be the namespace for all properties related to the 'statistics' feature.
The namespace identifies a manager that handles property-groups persisted as property-maps. For instance, all statistics pertaining to a given table would be collocated in the same property-group. As such, all properties (say number of 'unique_values' per columns) for a given HMS table 'relation0' would all be stored and persisted in the same property-map instance.
Property-maps may be decorated by an (optional) schema that may declare the name and value-type of allowed properties (and their optional default value). Each property is addressed by a name, a path uniquely identifying the property in a given property map.
The manager also handles transforming property-map names to the property-map keys used to persist them in the DB.

The API provides inserting/updating properties in bulk transactionally. It also provides selection/projection to help reduce the volume of exchange between client/server; selection can use (JEXL expression) predicates to filter maps.