impacts VM-wide behavior.

 *  New: Reduce contention for applications that make a very high number of concurrent requests.
    Previously OkHttp used its connection pool as a lock when making changes to connections and
    calls. With this change each connection is locked independently.

 *  Upgrade: [Okio 2.7.0][okio_2_7_0].

    ```kotlin

    lock.assertNotHeld()

    val connection = this.connection
    if (connection != null) {
      connection.lock.assertNotHeld()
      val toClose: Socket? =
        connection.withLock {
          // Sets this.connection to null.
          releaseConnectionNoEvents()
        }
      if (this.connection == null) {
        toClose?.closeQuietly()

import okhttp3.internal.ws.RealWebSocket
import okio.BufferedSink
import okio.BufferedSource

/**
 * A connection to a remote web server capable of carrying 1 or more concurrent streams.
 *
 * Connections are shared in a connection pool. Accesses to the connection's state must be guarded
 * by holding a lock on the connection.
 */
class RealConnection(
  val taskRunner: TaskRunner,
  val connectionPool: RealConnectionPool,

      val logHeaders = logBody || level == Level.HEADERS

      val requestBody = request.body

      val connection = chain.connection()
      var requestStartMessage =
        ("--> ${request.method} ${redactUrl(request.url)}${if (connection != null) " " + connection.protocol() else ""}")
      if (!logHeaders && requestBody != null) {
        requestStartMessage += " (${requestBody.contentLength()}-byte body)"

    server.bodyLimit = 7 * 512 * 1024
    val connection = server.url("/").toUrl().openConnection() as HttpURLConnection
    connection.setRequestMethod("POST")
    connection.setDoOutput(true)
    connection.setFixedLengthStreamingMode(1024 * 1024 * 1024) // 1 GB
    connection.connect()
    val out = connection.outputStream
    val data = ByteArray(1024 * 1024)
    var i = 0
    while (i < 1024) {

    // 3 different lookups in the connection pool!
    val pooled3 = routePlanner.planReusePooledConnection(this, routes)
    if (pooled3 != null) return pooled3.connection

    connection.withLock {
      connectionPool.put(connection)
      user.acquireConnectionNoEvents(connection)
    }

    user.connectionAcquired(connection)
    user.connectionConnectionAcquired(connection)
    return connection
  }

            clientTestRule.wrap(
              object : EventListener() {
                override fun connectionAcquired(
                  call: Call,
                  connection: Connection,
                ) {
                  socketClass = connection.socket().javaClass.name
                }
              },
            ),
          )

 *     sha256/AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
 *   at okhttp3.CertificatePinner.check(CertificatePinner.java)
 *   at okhttp3.Connection.upgradeToTls(Connection.java)
 *   at okhttp3.Connection.connect(Connection.java)
 *   at okhttp3.Connection.connectAndSetOwner(Connection.java)
 * ```
 *
 * Follow up by pasting the public key hashes from the exception into the
 * certificate pinner's configuration:
 *
 * ```java

      .assertLogEqual("<-- END HTTP")
      .assertNoMoreLogs()
    networkLogs
      .assertLogEqual("--> GET $url http/1.1")
      .assertLogEqual("Host: $host")
      .assertLogEqual("Connection: Keep-Alive")
      .assertLogEqual("Accept-Encoding: gzip")
      .assertLogMatch(Regex("""User-Agent: okhttp/.+"""))
      .assertLogEqual("--> END GET")
      .assertLogMatch(Regex("""<-- 200 OK $url \(\d+ms\)"""))

      }

    /**
     * Attempts to perform a web socket upgrade on the connection.
     * This will overwrite any previously set status, body, or streamHandler.
     */
    fun webSocketUpgrade(listener: WebSocketListener) =
      apply {
        status = "HTTP/1.1 101 Switching Protocols"
        setHeader("Connection", "Upgrade")
        setHeader("Upgrade", "websocket")
        webSocketListener = listener