Code cleanup (#199)

- Propagate @deprecated annotation to implementations
- Remove redundant "static final" definition from interface
- Use Collections.emptyMap instead of Collections.EMPTY_MAP to preserve type

opentracing-mock/src/main/java/io/opentracing/mock/MockTracer.java

@@ -263,6 +263,7 @@ public SpanBuilder withStartTimestamp(long microseconds) {
         }
 
         @Override
+        @Deprecated
         public MockSpan start() {
             return startManual();
         }

opentracing-noop/src/main/java/io/opentracing/noop/NoopSpanBuilder.java

@@ -23,7 +23,7 @@
 import java.util.Map;
 
 public interface NoopSpanBuilder extends Tracer.SpanBuilder, NoopSpanContext {
-    static final NoopSpanBuilder INSTANCE = new NoopSpanBuilderImpl();
+    NoopSpanBuilder INSTANCE = new NoopSpanBuilderImpl();
 }
 
 final class NoopSpanBuilderImpl implements NoopSpanBuilder {
@@ -67,6 +67,7 @@ public Tracer.SpanBuilder withStartTimestamp(long microseconds) {
     }
 
     @Override
+    @Deprecated
     public Span start() {
         return startManual();
     }
@@ -83,7 +84,7 @@ public Span startManual() {
 
     @Override
     public Iterable<Map.Entry<String, String>> baggageItems() {
-        return Collections.EMPTY_MAP.entrySet();
+        return Collections.<String, String>emptyMap().entrySet();
     }
 
     @Override