Do not start write to connection if blocked on bw

Tor can to start write to connection just because something wrote to conn's outbuf by _connection_write_to_buf_impl(). While connection can be blocked on bw still. If buckets empty it can't to write anything anyway (need investigation for sure).

Suggested idea to fix, could be looks like:

  if (conn->write_event && !conn->write_blocked_on_bw) {
    connection_start_writing(conn);
  }

Kludgy probably. legacy/trac#8111 (moved) will fix it generally for 0.2.5 I hopes.