Use Discord heartbeat interval in audio. (#2765)

Reduce heartbeat interval to ensure Discord receives it within range.
Refactor some AudioClient code.

Co-authored-by: Quin Lynch <49576606+quinchs@users.noreply.github.com>

src/Discord.Net.Core/DiscordConfig.cs

@@ -238,6 +238,15 @@ namespace Discord
         /// </summary>
         public const int MaxApplicationTagCount = 5;
 
+        /// <summary>
+        ///     Returns the factor to reduce the heartbeat interval.
+        /// </summary>
+        /// <remarks>
+        ///     If a heartbeat takes longer than the interval estimated by Discord, the connection will be closed.
+        ///     This factor is used to reduce the interval and ensure that Discord will get the heartbeat within the estimated interval.
+        /// </remarks>
+        internal const double HeartbeatIntervalFactor = 0.9;
+
         /// <summary>
         ///     Returns the maximum length of a voice channel status.
         /// </summary>