MDL-19711 dml: Enable use of readonly slave database handles

Implemented with moodle_read_slave_trait

Functionality is triggered by supplying config dboption['readonly'].
See config-dist.php for more info on supported dboptions.

pgsql and mysqli drivers are using this feature. Also added support for
connection timeout for these two drivers.

lib/dml/moodle_database.php

@@ -108,13 +108,13 @@ abstract class moodle_database {
     /** @var float Last time in seconds with millisecond precision. */
     protected $last_time;
     /** @var bool Flag indicating logging of query in progress. This helps prevent infinite loops. */
-    private $loggingquery = false;
+    protected $loggingquery = false;
 
     /** @var bool True if the db is used for db sessions. */
     protected $used_for_db_sessions = false;
 
     /** @var array Array containing open transactions. */
-    private $transactions = array();
+    protected $transactions = array();
     /** @var bool Flag used to force rollback of all current transactions. */
     private $force_rollback = false;
 
@@ -2717,6 +2717,22 @@ abstract class moodle_database {
         return $this->reads;
     }
 
+    /**
+     * Returns whether we want to connect to slave database for read queries.
+     * @return bool Want read only connection
+     */
+    public function want_read_slave(): bool {
+        return false;
+    }
+
+    /**
+     * Returns the number of reads before first write done by this database.
+     * @return int Number of reads.
+     */
+    public function perf_get_reads_slave(): int {
+        return 0;
+    }
+
     /**
      * Returns the number of writes done by this database.
      * @return int Number of writes.