table resolution".
Added members to Item_ident for storing original db, table and field
names since those that set later from Field have shorter life-time 
than required by prep. stmt. So we need to restore original names in 
Item_ident::cleanup(). Also now using special construnctor for creation
of Item_field from Field object that ensures that table and field name 
have big enough life-time.

"Fix" for bug #2050 "10 to 1 performance drop with server 4.1.1"
Clean ups in implementation of caching of field number in table.
Added caching of table in which field is found in find_field_in_tables().