Deprecate MasterSlaveConnection class

Not only is the terminology needlessly hurtful, the design is a very
leaky abstraction, and users are better off using regular connections
and managing them themselves explicitly.

UPGRADE.md

@@ -1,5 +1,10 @@
 # Upgrade to 2.11
 
+## Deprecated `MasterSlaveConnection`
+
+The naming is offensive, the implementation very questionable. Use each
+endpoint connection explicitly.
+
 ## Deprecated `ArrayStatement` and `ResultCacheStatement` classes.
 
 The `ArrayStatement` and `ResultCacheStatement` classes are deprecated. In a future major release they will be renamed and marked internal as implementation details of the caching layer.

lib/Doctrine/DBAL/Connections/MasterSlaveConnection.php

@@ -15,6 +15,10 @@
 use function assert;
 use function count;
 use function func_get_args;
+use function sprintf;
+use function trigger_error;
+
+use const E_USER_DEPRECATED;
 
 /**
  * Master-Slave Connection
@@ -53,6 +57,8 @@
  *
  * Instantiation through the DriverManager looks like:
  *
+ * @deprecated use regular connections instead
+ *
  * @example
  *
  * $conn = DriverManager::getConnection(array(
@@ -93,6 +99,11 @@ class MasterSlaveConnection extends Connection
      */
     public function __construct(array $params, Driver $driver, ?Configuration $config = null, ?EventManager $eventManager = null)
     {
+        @trigger_error(sprintf(
+            'Class "%s" is deprecated since doctrine/dbal 2.11 and will be removed in 3.0',
+            self::class
+        ), E_USER_DEPRECATED);
+
         if (! isset($params['slaves'], $params['master'])) {
             throw new InvalidArgumentException('master or slaves configuration missing');
         }