Lookup in Transforms

Requirements

1. Operations
   1. Perform single + batch read on single + multiple dataset from script transform
   2. Perform single + batch read on DistributedCache from script transform
2. Supported datasets for lookup
   1. Key-value table
   2. ObjectMappedTable
3. Optional caching with time-based expiration

Design

1. LookupKV interface 

   Object lookup(String key);
   Map<String, Object> multiLookup(String[] key);

2. Implement LookupKV in KeyValueTable and ObjectMappedTable
3. ScriptTransform changes
   1. Add configuration property for declaring lookup tables to use, properties for each table (e.g. dataset properties)

      1. Example

         [
           {"name":"purchases", "dataset":"purchases", "datasetProperties": {..}, "enable.cache":"true", "cache.expiry":1234},
           {"name":"ip2geo", "file":"/data/ip2geo.csv"}
         ]

   2. configure(): verify datasets / tables exist
   3. transform(): execute lookup methods in a transaction, provide LookupKV instance to script
      1. Sample usage: context.getTable("purchases").lookup(user)
      2. Alternative: tables["purchases"].lookup(user)
      3. Alternative: purchases.lookup(user)

      4. Sample usage for multiLookup:

         var result = purchases.multiLookup(["alice", "bob"]);
         // do something with result["alice"]
         // do something with result["bob"]