crash on backend restart if persistent connections are in use

1. Create 75 connections to each service: RW split, ReadConn master, ReadConn slave with Master/Slave backend, RW split with Galera backend
2. Close all connections
3. Stop and restart backends

4. Check logs and cores

Expected results: no crash

Actual result: crash during Maxscale restart

(gdb) bt
#0 0x00007f8a59354ced in router_handle_state_switch (dcb=0x1f504a0, reason=DCB_REASON_NOT_RESPONDING, data=0x1f4af50)
at /home/ec2-user/workspace/server/modules/routing/readwritesplit/readwritesplit.c:5310
#1 0x0000000000557cd0 in dcb_call_callback (dcb=0x1f504a0, reason=DCB_REASON_NOT_RESPONDING) at /home/ec2-user/workspace/server/core/dcb.c:2629
#2 0x0000000000557f19 in dcb_call_foreach (server=0x1f3bba0, reason=DCB_REASON_NOT_RESPONDING) at /home/ec2-user/workspace/server/core/dcb.c:2743
#3 0x00007f8a53de9b25 in monitorMain (arg=0x1f4fcf0) at /home/ec2-user/workspace/server/modules/monitor/mysql_mon.c:864
#4 0x00007f8a733a8df3 in start_thread () from /lib64/libpthread.so.0
#5 0x00007f8a71c7901d in clone () from /lib64/libc.so.6
(gdb)