Make the baker preendorse as soon as it receives a valid block

Context

This MR changes the baker semantics so that instead of waiting for a block to be fully applied before emitting a preendorsement, it waits for a block to be considered valid by the node. This significantly reduces the delay of reaching a Tenderbake prequorum. The baker still waits for the block to be applied in order to lock itself on a value that reached a prequorum and, thus, also waits before endorsing for the quorum.

Manually testing the MR

Bake a block that takes a while to apply, notice that the baker starts preendorsing as soon as the block is considered valid by the node but still waits for the block to be applied to start endorsing.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR