MySQL backend generates foreign keys over length limit

Currently, persistent-mysql suggests foreign key constraint names that are over MySQL's length limit of 64 characters, which MySQL will throw an error for.

persistent-mysql should generate truncated names that don't go over the length limit. It could use the same logic as is introduced in #996 for truncating Postgres keys, or logic that better matches MySQL's naming conventions.

This issue is discussed in greater detail in #996, which adds tests for it. This issue is mostly to track the issue being resolved for MySQL, since #996 will be closed after it resolves it for Postgres.