After FLUSH STATUS max_used_connections was reset to 0, and haven't
been updated while cached threads were reused, until the moment a new
thread was created.

The first suggested fix from original bug report was implemented:

  a) On flushing the status, set max_used_connections to
  threads_connected, not to 0.

  b) Check if it is necessary to increment max_used_connections when
  taking a thread from the cache as well as when creating new threads