TS-1679: channel_stats: count 5xx response and clean up code
TS-1252: stats summary in cluster not working

Here is some stats with evil result, we all know that most stats
with proxy.process.cluster prefix have no result at all.

   proxy.process.cluster.open_delays=-4411607530454032024
   proxy.process.cluster.control_messages_avg_send_time=-72.902885
   proxy.process.cluster.open_delay_time=-2494.564697
   proxy.process.cluster.rmt_cache_callback_time=-747.036865
   proxy.process.cluster.local_connection_time=-15.002351

When int64_t integer was passed to CLUSTER_SUM_DYN_STAT(x, y) as
2th parameter, it would be convert to int unsafely.
TS-1678: Simplify register_record()

'release_record_lock' parameter to register_record() is unnecessarily
complicated and the lock release is dangerous. Let's simplify it
and remove verbose lock releasing from the upper layer code.