Upgrade to error-prone 2.5.1

This addresses issues seen with more recent Java and Maven versions.
Enabled by requiring Java 8.  Also address and suppress a few stray
warnings.

pom.xml

@@ -94,30 +94,31 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-compiler-plugin</artifactId>
-        <version>3.6.1</version>
+        <version>3.8.0</version>
         <configuration>
-          <compilerId>javac-with-errorprone</compilerId>
-          <forceJavacCompilerUse>true</forceJavacCompilerUse>
           <source>${java.version}</source>
           <target>${java.version}</target>
           <showDeprecation>true</showDeprecation>
           <showWarnings>true</showWarnings>
-          <compilerArguments>
-            <Xlint />
-          </compilerArguments>
+          <compilerArgs>
+            <arg>-Xlint</arg>
+            <arg>-XDcompilePolicy=simple</arg>
+            <arg>-Xplugin:ErrorProne
+                 -Xep:DefaultCharset:OFF
+                 -Xep:HidingField:OFF
+                 -Xep:JavaUtilDate:OFF
+                 -Xep:MutableConstantField:OFF
+                 -Xep:ProtectedMembersInFinalClass:OFF
+            </arg>
+          </compilerArgs>
+          <annotationProcessorPaths>
+            <path>
+              <groupId>com.google.errorprone</groupId>
+              <artifactId>error_prone_core</artifactId>
+              <version>2.5.1</version>
+            </path>
+          </annotationProcessorPaths>
         </configuration>
-        <dependencies>
-          <dependency>
-            <groupId>org.codehaus.plexus</groupId>
-            <artifactId>plexus-compiler-javac-errorprone</artifactId>
-            <version>2.6</version>
-          </dependency>
-          <dependency>
-            <groupId>com.google.errorprone</groupId>
-            <artifactId>error_prone_core</artifactId>
-            <version>2.0.5</version>
-          </dependency>
-        </dependencies>
       </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>

src/main/java/org/gaul/s3proxy/ChunkedInputStream.java

@@ -34,6 +34,7 @@ final class ChunkedInputStream extends FilterInputStream {
     @edu.umd.cs.findbugs.annotations.SuppressFBWarnings(
             value = "URF_UNREAD_FIELD",
             justification = "https://github.com/gaul/s3proxy/issues/205")
+    @SuppressWarnings("UnusedVariable")
     private String currentSignature;
 
     ChunkedInputStream(InputStream is) {

src/main/java/org/gaul/s3proxy/EventualBlobStore.java

@@ -197,6 +197,7 @@ final class EventualBlobStore extends ForwardingBlobStore {
         return part;
     }
 
+    @SuppressWarnings("FutureReturnValueIgnored")
     private void schedule(Callable<?> callable) {
         if (random.nextDouble() < probability) {
             deque.add(callable);

src/main/java/org/gaul/s3proxy/S3Proxy.java

@@ -425,12 +425,11 @@ public final class S3Proxy {
                             that.keyStorePassword) &&
                     Objects.equals(this.virtualHost, that.virtualHost) &&
                     Objects.equals(this.servicePath, that.servicePath) &&
-                    Objects.equals(this.maxSinglePartObjectSize,
-                            that.maxSinglePartObjectSize) &&
-                    Objects.equals(this.v4MaxNonChunkedRequestSize,
-                            that.v4MaxNonChunkedRequestSize) &&
-                    Objects.equals(this.ignoreUnknownHeaders,
-                            that.ignoreUnknownHeaders) &&
+                    this.maxSinglePartObjectSize ==
+                            that.maxSinglePartObjectSize &&
+                    this.v4MaxNonChunkedRequestSize ==
+                            that.v4MaxNonChunkedRequestSize &&
+                    this.ignoreUnknownHeaders == that.ignoreUnknownHeaders &&
                     this.corsRules.equals(that.corsRules);
         }