MDEV-27767 poor scaling with InnoDB and utf8mb3 because of charset stats

Access the all_charsets[] array directly in a hot loop.
This avoids get_charset() that increments a shared counter via
my_collation_statistics_inc_use_count(), causing a scalability issue.

Instead, call get_charset() when a table is opened (and InnoDB
fills in dtype_t values) - this is enough, as charset is marked
ready (MY_CS_READY) only once, on the first get_charset() call,
after that it can be accessed directly.

This also fixes a potential bug in InnoDB. It used to access charsets
directly when filling dtype_t values. This wasn't preceded by any
get_charset() call inside InnoDB. Normally the server would call
get_charset() on reading the frm, but if the table was first opened
from a purge thread InnoDB could, theoretically, access a not-ready
charset in innobase_get_cset_width().

storage/innobase/handler/ha_innodb.cc

@@ -2418,7 +2418,7 @@ innobase_get_cset_width(
 	ut_ad(mbminlen);
 	ut_ad(mbmaxlen);
 
-	cs = all_charsets[cset];
+	cs = cset ? get_charset((uint)cset, MYF(MY_WME)) : NULL;
 	if (cs) {
 		*mbminlen = cs->mbminlen;
 		*mbmaxlen = cs->mbmaxlen;

storage/innobase/rem/rem0cmp.cc

@@ -281,15 +281,13 @@ static int cmp_data(ulint mtype, ulint prtype, const byte *data1, ulint len1,
     /* fall through */
   case DATA_VARMYSQL:
     DBUG_ASSERT(is_strnncoll_compatible(prtype & DATA_MYSQL_TYPE_MASK));
-    if (CHARSET_INFO *cs= get_charset(dtype_get_charset_coll(prtype),
-                                      MYF(MY_WME)))
+    if (CHARSET_INFO *cs= all_charsets[dtype_get_charset_coll(prtype)])
       return cs->coll->strnncollsp(cs, data1, len1, data2, len2);
   no_collation:
     ib::fatal() << "Unable to find charset-collation for " << prtype;
   case DATA_MYSQL:
     DBUG_ASSERT(is_strnncoll_compatible(prtype & DATA_MYSQL_TYPE_MASK));
-    if (CHARSET_INFO *cs= get_charset(dtype_get_charset_coll(prtype),
-                                      MYF(MY_WME)))
+    if (CHARSET_INFO *cs= all_charsets[dtype_get_charset_coll(prtype)])
       return cs->coll->strnncollsp_nchars(cs, data1, len1, data2, len2,
                                           std::max(len1, len2));
     goto no_collation;