Remove tracking of log_position

This patch removes log_position from streaming_context. The
log_position was meant for keeping track of the database specific
position corresponding to the changes that have been already
replicated by a streaming transaction. In reality, things may be more
complex in the DBMS side and a size_t may be unsufficient to keep
track of the progress of a streaming transaction. For example, in
MariaDB, it may be necessary to keep track of positions in both
transaction and statement caches. Suggesting that the responsibility
of tracking these position(s) should be delegated to client_service.
The log_position was also used to do sanity checks in
streaming_step(). Those sanity checks are preserved by simply keeping
track of the number of bytes that were certified by the streaming
transaction.

dbsim/db_client_service.hpp

@@ -51,10 +51,8 @@ namespace db
         {
             return true;
         }
-        int prepare_fragment_for_replication(wsrep::mutable_buffer&,
-                                             size_t& position) override
+        int prepare_fragment_for_replication(wsrep::mutable_buffer&) override
         {
-            position = 0;
             return 0;
         }
         int remove_fragments() override { return 0; }