Adding the cache and the retriver stubs. (#215)

* Adding the cache and the retriver stubs.

* Addressing comments and adding initial documentation and changing the retreiver from interface to class.

* Reverting back to the interface

* Fixing minor error

* Adding default cache stub

src/main/java/com/amazonaws/services/kinesis/clientlibrary/lib/worker/DefaultGetRecordsCache.java

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+package com.amazonaws.services.kinesis.clientlibrary.lib.worker;
+
+import java.util.Optional;
+import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
+
+import com.amazonaws.services.kinesis.model.GetRecordsResult;
+
+import lombok.NonNull;
+import lombok.extern.apachecommons.CommonsLog;
+
+/**
+ *
+ */
+@CommonsLog
+public class DefaultGetRecordsCache implements GetRecordsCache {
+    private static final int DEFAULT_MAX_SIZE = 1;
+    private static final int DEFAULT_MAX_BYTE_SIZE = 1;
+    private static final int DEFAULT_MAX_RECORDS_COUNT = 1;
+    
+    private final Queue<GetRecordsResult> getRecordsResultQueue;
+    private final int maxSize;
+    private final int maxByteSize;
+    private final int maxRecordsCount;
+    private final int maxRecordsPerCall;
+    @NonNull
+    private final GetRecordsRetrievalStrategy getRecordsRetrievalStrategy;
+
+    public DefaultGetRecordsCache(final Optional<Integer> maxSize, final Optional<Integer> maxByteSize, final Optional<Integer> maxRecordsCount,
+                                  final int maxRecordsPerCall, final GetRecordsRetrievalStrategy getRecordsRetrievalStrategy) {
+        this.getRecordsResultQueue = new ConcurrentLinkedQueue<>();
+        this.maxSize = maxSize.orElse(DEFAULT_MAX_SIZE);
+        this.maxByteSize = maxByteSize.orElse(DEFAULT_MAX_BYTE_SIZE);
+        this.maxRecordsCount = maxRecordsCount.orElse(DEFAULT_MAX_RECORDS_COUNT);
+        this.maxRecordsPerCall = maxRecordsPerCall;
+        this.getRecordsRetrievalStrategy = getRecordsRetrievalStrategy;
+    }
+
+    @Override
+    public GetRecordsResult getNextResult() {
+        return null;
+    }
+
+    @Override
+    public void shutdown() {
+
+    }
+}

src/main/java/com/amazonaws/services/kinesis/clientlibrary/lib/worker/GetRecordsCache.java

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+package com.amazonaws.services.kinesis.clientlibrary.lib.worker;
+
+import com.amazonaws.services.kinesis.model.GetRecordsResult;
+
+/**
+ * This class is used as a cache for Prefetching data from Kinesis.
+ */
+public interface GetRecordsCache {
+    /**
+     * This method returns the next set of records from the Cache if present, or blocks the request till it gets the
+     * next set of records back from Kinesis.
+     * 
+     * @return The next set of records.
+     */
+    GetRecordsResult getNextResult();
+    
+    void shutdown();
+}

src/main/java/com/amazonaws/services/kinesis/clientlibrary/lib/worker/GetRecordsRetriever.java

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+package com.amazonaws.services.kinesis.clientlibrary.lib.worker;
+
+import com.amazonaws.services.kinesis.model.GetRecordsResult;
+
+import java.util.concurrent.Callable;
+
+/**
+ * This class uses the GetRecordsRetrievalStrategy class to retrieve the next set of records and update the cache. 
+ */
+public interface GetRecordsRetriever {
+    GetRecordsResult getNextRecords(int maxRecords);
+}