MySQL calls external_lock at the beginning and end of a statement
when it is not calling start_stmt or commit or rollback.  Thus,
statement boundaries can be (and are already) detected without
monitoring thd->query_id.

The function innobase_commit() seemingly lacks the call to
innobase_release_stat_resources(), which should be called at
the end of every SQL statement.  The call was replaced by
equivalent statements by Vadim Tkachenko when he implemented
innodb_commit_concurrency in MySQL 5.0:

http://mysql.bkbits.net:8080/mysql-5.0/?PAGE=patch&REV=1.1886.70.1