Enable Jest's multi-threaded support by default

Since the `JestClient` is auto-configured as a bean, it must have its
multi-threaded support enabled by default. This commit exposes a new
`spring.elasticsearch.jest.multi-threaded` property that is `true` by
default.

Closes gh-6806

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/elasticsearch/jest/JestAutoConfiguration.java

@@ -81,6 +81,7 @@ public class JestAutoConfiguration {
 		if (gson != null) {
 			builder.gson(gson);
 		}
+		builder.multiThreaded(this.properties.isMultiThreaded());
 		return builder.connTimeout(this.properties.getConnectionTimeout())
 				.readTimeout(this.properties.getReadTimeout()).build();
 	}

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/elasticsearch/jest/JestProperties.java

@@ -45,6 +45,11 @@ public class JestProperties {
 	 */
 	private String password;
 
+	/**
+	 * Enable connection requests from multiple execution threads.
+	 */
+	private boolean multiThreaded = true;
+
 	/**
 	 * Connection timeout in milliseconds.
 	 */
@@ -84,6 +89,14 @@ public class JestProperties {
 		this.password = password;
 	}
 
+	public boolean isMultiThreaded() {
+		return this.multiThreaded;
+	}
+
+	public void setMultiThreaded(boolean multiThreaded) {
+		this.multiThreaded = multiThreaded;
+	}
+
 	public int getConnectionTimeout() {
 		return this.connectionTimeout;
 	}

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -631,6 +631,7 @@ content into your application; rather pick only the properties that you need.
 
 	# JEST (Elasticsearch HTTP client) ({sc-spring-boot-autoconfigure}/jest/JestProperties.{sc-ext}[JestProperties])
 	spring.elasticsearch.jest.connection-timeout=3000 # Connection timeout in milliseconds.
+	spring.elasticsearch.jest.multi-threaded=true # Enable connection requests from multiple execution threads.
 	spring.elasticsearch.jest.password= # Login password.
 	spring.elasticsearch.jest.proxy.host= # Proxy host the HTTP client should use.
 	spring.elasticsearch.jest.proxy.port= # Proxy port the HTTP client should use.