buf_dblwr unnecessarily uses memory-intensive srv_stats counters

After the code refactoring that was performed in MDEV-23855, we can replace the use of srv_stats counters in buf_dbwlr with regular variables that are protected by buf_dblwr.mutex.

The counters in srv_stats use std::atomic and multiple cache lines per counter. This is an overkill in a case where a critical section already exists in the code. A regular variable will work just fine, with much smaller memory bus impact.