boost: draft for vtgate boost query checks

[benetis]

Following on our sync:

I've added boost config to be part of the engine.Plan. So in https://github.com/vinted/vitess/pull/8/files#diff-19aa815da9f21f65b55cc865090e2cefa77af98b30b134936853f7763898d255R123 we can do dirty check if its boosted and do redis query instead. However, since whole plan is available – we can also continue with vitess query. And since this function returns query result itself in last line – I think it could work as we would save the result in variable, put it in redis and then return result itself.
I think for a hacky quick solution – could be doable

Draft version, no tests and so on to enable us working together on this