Temporal client accounting

[krizhanovsky]

The task is from #488 (comment):

From #100: the most urgent thing is to keep security accounting data for a client for some time after the last client connection is closed. This is very important to track client security limits properly for Connection: closed connections. See https://github.com/tempesta-tech/tempesta/blob/master/tempesta_fw/client.c#L89

Since we have to evict client accounting data after 'some time', it has sense to store them in a TDB table. At this time TDB has issues with eviction - just let's leave with them untill they are fixed in Beta.

Next, the hash key for searching a client must be calculated by User-Agent plus IP address, otherwise, if no User-Agent, only IP address.

Currently, we use netowork IP address as the address of a client, however if a client work through a forward proxy, then a proxy can pass it's IP address by the first item in X-Forwarded-For, so if the header is present, then we shall reinsert the TfwClient in TDB with different key.

The reinsert operation must be implemented on TDB layer as a new routine tdb_entry_reinsert() accepting current and new keys. The function must call tdb_htrie_insert(), copying the data from the previous location, and a new empty tdb_htrie_delete() left as TODO for #515.

Traversal all the clients on shutdown must be done by a new TDB routine tdb_walk() which is just a walk on the radix tree plus tdb_rec_next() call. The routine must expect that there is no tree writers (the operation must be done after closing the listener socket and sycnrhonization barrier on all softirqs).