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)

      "lookupTables": [
        {"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. Options usage for lookup usage: 

         var result = context.getTable("purchases").lookup(user);
         // or
         var result = tables["purchases"].lookup(user);
         // or
         var result = purchases.lookup(user);

      2. Options for multiLookup usage:

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