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().