Refactor `validate_block` (#2069)

* Refactor `validate_block`

This pull request changes the `validate_block` implementation. One of the key changes are that we
free data structures as early as possible. The memory while validating the block is scarce and we
need to give as much as possible to the actual execution of the block. Besides that the pr moves the
validation of the `validation_data` into the `validate_block` implementation completely instead of
using this machinery with putting the data into some global variable that would then be read while
executing the block. There are also some new docs to explain the internals of `validate_block`.

* No clone wars!!

* Integrate more feedback

* FMT

* Delay the header encoding