eventstat can segfault when reaping stale timer stats

user has seen the following segfault:

eventstat[10865]: segfault at af7 ip 00007f9d821ff872 sp 00007ffdb479a870 error 4 in libc-2.21.so[7f9d8217b000+1c0000]

stale timer stats are periodically reaped to keep memory footprint low, however, there are two issues with this:

1. The hash stat removal was double referencing the timer stat linked list next pointer instead of the hash linked list next pointer
2. More than one timer info can reference a timerstat, so we should only remove the stale timer state when no more timer infos reference it (e.g. we need to add reference counting).