Fix Netty 4.2.7 compatibility by using string literals for HTTP headers

Replace Netty HttpHeaderNames and HttpHeaderValues constants with plain
string literals to avoid compatibility issues with Netty 4.2.7.Final.

Changes:
- Replace HttpHeaderNames.CONTENT_TYPE with "Content-Type"
- Replace HttpHeaderNames.CONTENT_ENCODING with "Content-Encoding"
- Replace HttpHeaderValues.GZIP with "gzip"
- Remove unused Netty header imports

This avoids any potential incompatibilities between AsyncHttpClient 3.0.3
and Netty 4.2.7's header constant implementations.

Author: claude

src/main/java/com/arpnetworking/tsdcore/sinks/HttpPostSink.java

@@ -25,8 +25,6 @@
 import com.fasterxml.jackson.annotation.JacksonInject;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Lists;
-import io.netty.handler.codec.http.HttpHeaderNames;
-import io.netty.handler.codec.http.HttpHeaderValues;
 import net.sf.oval.Validator;
 import net.sf.oval.constraint.CheckWith;
 import net.sf.oval.constraint.CheckWithCheck;
@@ -127,12 +125,12 @@ protected RequestInfo createRequest(final AsyncHttpClient client, final byte[] s
 
         final RequestBuilder requestBuilder = new RequestBuilder()
                 .setUri(getAysncHttpClientUri())
-                .setHeader(HttpHeaderNames.CONTENT_TYPE, MediaTypes.APPLICATION_JSON.toString())
+                .setHeader("Content-Type", MediaTypes.APPLICATION_JSON.toString())
                 .setBody(bodyData)
                 .setMethod(HttpConstants.Methods.POST);
 
         if (_enableCompression) {
-            requestBuilder.setHeader(HttpHeaderNames.CONTENT_ENCODING, HttpHeaderValues.GZIP);
+            requestBuilder.setHeader("Content-Encoding", "gzip");
         }
 
         return new RequestInfo(requestBuilder.build(), serializedData.length, bodyData.length);

src/main/java/com/arpnetworking/tsdcore/sinks/KairosDbSink.java

@@ -35,7 +35,6 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Lists;
-import io.netty.handler.codec.http.HttpHeaderNames;
 import it.unimi.dsi.fastutil.doubles.Double2LongMap;
 import net.sf.oval.constraint.Min;
 import net.sf.oval.constraint.NotNull;
@@ -177,7 +176,7 @@ protected RequestInfo createRequest(final AsyncHttpClient client, final byte[] s
 
             final RequestBuilder requestBuilder = new RequestBuilder()
                     .setUri(getAysncHttpClientUri())
-                    .setHeader(HttpHeaderNames.CONTENT_TYPE, "application/gzip")
+                    .setHeader("Content-Type", "application/gzip")
                     .setBody(bodyData)
                     .setMethod(HttpConstants.Methods.POST);