Add SHOW STATUS counters for temp tables

Add two new counters:

Tmp_table_bytes_used: the number of bytes written to temp tables.

Tmp_table_max_size: the maximum size any temp table has grown to, in bytes. (I cannot think of a good name for this one that isn't easy to confuse with max_temp_table_size, which already exists. Maybe there is a better name.)

Both of these can be global and session.