detect/threshold: implement per thread cache

detect/threshold: implement per thread cache

Thresholding often has 2 stages:

1. recording matches
2. appling an action, like suppress

E.g. with something like:
threshold:type limit, count 10, seconds 3600, track by_src;
the recording state is about counting 10 first hits for an IP,
then followed by the "suppress" state that might last an hour.

By_src/by_dst are expensive, as they do a host table lookup and lock
the host. If many threads require this access, lock contention becomes
a serious problem.

This patch adds a thread local cache to avoid the synchronization
overhead. When the threshold for a host enters the "apply" stage,
a thread local hash entry is added. This entry knows the expiry
time and the action to apply. This way the action can be applied
w/o the synchronization overhead.

A rbtree is used to handle expiration.

Implemented for IPv4.