Remove concept of multiple timer managers

- All timers are now handled by a single global timer manager, which simplifies how they handled by the IOSource manager.
- This change flows down a number of changes to other parts of the code. The timer manager tag field is removed, which means that matching connections to a timer manager is also removed. This removes the ability to tag a connection as internal or external, since that's how the connections where differentiated. This in turn removes the `current_conns_extern` field from the `ConnStats` record type in the script layer.

src/iosource/IOSource.h

@@ -70,17 +70,6 @@ public:
 	 */
 	virtual void Process() = 0;
 
-	/**
-	 * Returns the tag of the timer manager associated with the last
-	 * procesees data item.
-	 *
-	 * Can be overridden by derived classes.
-	 *
-	 * @return The tag, or null for the global timer manager.
-	 *
-	 */
-	virtual std::string* GetCurrentTag()	{ return 0; }
-
 	/**
 	 * Returns a descriptive tag representing the source for debugging.
 	 *