Move the preapply RPC's backend to the validator process

Context

This MR moves the preapply to the validator process. This MR also contains necessary refactoring that harmonizes the Block_validation API. This allows the shell to cache the preapplication result in order to only commit when validating the same block. The resulting effect is that it divides by (roughly) 2 the time spent preapplying then validating a same block. In particular, this is the case for migration blocks that are baked by the node. This is necessary for Hangzhou's migration which will be highly time consuming.