Problem:

sp_cache erroneously looked up fully qualified SP names (e.g. `DB`.`SP`),
in case insensitive style. It was wrong, because only the "name"
part is always case insensitive, while the "db" part should be compared
according to lower_case_table_names (case sensitively for 0,
case insensitively for 1 and 2).

Fix:

Adding a "casedn_name" parameter make_qname() to tell
if the name part should be lower cased:
  `DB1`.`SP` -> "DB1.SP"  (when casedn_name=false)
  `DB1`.`SP` -> "DB1.sp"  (when casedn_name=true)
and using make_qname() with casedn_name=true when creating
sp_cache hash lookup keys.

Details:

As a result, it now works as follows:
- sp_head::m_db is converted to lower case if lower_case_table_names>0
  during the sp_name initialization phase. So when make_qname() is called,
  sp_head::m_db is already normalized. There are no changes in here.

- The initialization phase of sp_head when creating sp_head::m_qname
  now calls make_qname() with casedn_name=true,
  so sp_head::m_name gets written to sp_head::m_qname in lower case.

- sp_cache_lookup() now also calls make_qname() with casedn_name=true,
  so sp_head::m_name gets written to the temporary lookup key in lower case.

- sp_cache::m_hashtable now uses case sensitive comparison