WL#4971: select_number need not be generated before it is used

Affects: WorkLog-3.4   —   Status: Un-Assigned   —   Priority: Medium

The select_number field inside st_select_lex is only required for EXPLAIN 
functionality. Hence, it can probably be generated locally inside 
select_describe() and related functions.

The select_number field in st_select_lex object and the global counter in thd/
lex can thus be eliminated.