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.

scripts/base/init-bare.zeek

@@ -548,7 +548,6 @@ type NetStats: record {
 type ConnStats: record {
 	total_conns: count;           ##<
 	current_conns: count;         ##<
-	current_conns_extern: count;  ##<
 	sess_current_conns: count;    ##<
 
 	num_packets: count;