issue #535 - Deduplicate the SearchParameter map

Previously, the parameter maps built by ParametersUtil had each
SearchParameter listed twice...once by "code" and once by "url".

This pull request introduces a new ParametersMap object that
encapsulates two separate maps and presents it as a single object.

Signed-off-by: Lee Surprenant <lmsurpre@us.ibm.com>

fhir-search/src/main/java/com/ibm/fhir/search/parameters/ParametersMap.java

@@ -0,0 +1,64 @@
+/*
+ * (C) Copyright IBM Corp. 2019
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package com.ibm.fhir.search.parameters;
+
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.Map;
+
+import com.ibm.fhir.model.resource.SearchParameter;
+
+/**
+ * A object for storing multiple on a common list of SearchParameter 
+ */
+public class ParametersMap {
+    private final Map<String, SearchParameter> codeMap;
+    private final Map<String, SearchParameter> urlMap;
+
+    /**
+     * Construct a ParametersMap from a Bunlde of SearchParameter
+     */
+    public ParametersMap() {
+        codeMap = new HashMap<>();
+        urlMap = new HashMap<>();
+    }
+
+    /**
+     * @implSpec package-private to prevent insertion from outside the package
+     */
+    void insert(String code, String url, SearchParameter parameter) {
+        codeMap.put(code, parameter);
+        urlMap.put(url, parameter);
+    }
+
+    /**
+     * @implSpec package-private to prevent insertion from outside the package
+     */
+    void insertAll(ParametersMap map) {
+        codeMap.putAll(map.codeMap);
+        urlMap.putAll(map.urlMap);
+    }
+
+    public SearchParameter lookupByCode(String searchParameterCode) {
+        return codeMap.get(searchParameterCode);
+    }
+
+    public SearchParameter lookupByUrl(String searchParameterUrl) {
+        return urlMap.get(searchParameterUrl);
+    }
+
+    public Collection<SearchParameter> values() {
+        return codeMap.values();
+    }
+
+    public boolean isEmpty() {
+        return codeMap.isEmpty();
+    }
+
+    public int size() {
+        return codeMap.size();
+    }
+}