MULTIPLE CONNECTION

The crash caused by concurrent get_charset_number_internal() calls is because
the std::unsorted_map::operator [] is not thread safe. When there are
multiple threads, one might access the memory already freed by another one.

There is also memory usage continuously increasing observed.  This is
because the collation name - collation id mapping allocs new buffer for new
added collations, even when the collation doesn't exist at all.

Fix:
Check the existence of collation when getting its id by the mapping. The
collation initialization should have been done in init_available_charsets()
which is thread-safe with std::call_once.

Change-Id: Ie604901a2dce5a1f1a794cdd2196bf5434f9202f