String key comparisons in the Performance Schema sometimes failed for columns
defined with charset = utf8mb4 because the max multibyte length was
assumed to be 3 rather than 4.

Replaced the artificial truncation of key strings that exceeded the buffer size with
a debug assert to ensure that the buffer capacity is sufficient for the given key.

Adjusted the buffer sizes of indexed fields in pfs_example_component and pfs_example_plugin
to account for a max multibyte length.

Reviewed by: Marc Alff <marc.alff@oracle.com>