Introduce @@optimizer_fixes variable

As discussed on Slack, we need to introduce @@optimizer_fixes variable.

It should be a bitmap of fixes that were added to this version but could not be enabled by default due to plan stability/risk concerns.

The difference from optimizer_switch is that optimizer_switch controls features. optimizer_switch features are expected to stay visible forever.

Optimizer fixes are expected to disappear (become always enabled) in some subsequent release.

We've already had a variable that is "a bitmap of fixes". It was optimizer_adjust_secondary_key_costs (poorly named after the first such fix).