Cache execution plans in PS and SP

MySQL and MariaDB never cache execution plans for prepared statements, the optimizer is run for every PS execution. Traditionally we've been saying that a table might change between two PS executions and these changes might be big enough to affect the statistics and the execution plan. But now when engine independent statistics is enabled this argument is no longer valid.

Note, the execution plan might still depend on exact values in placeholders, there are different ways of solving this issue (not cache these plans, cache the first plan, cache the "average" plan, etc).