Rework Transaction Priority calculation (#9834)

* Add transaction validity docs.

* Re-work priority calculation.

* Fix tests.

* Update frame/transaction-payment/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* cargo +nightly fmt --all

* Fix an obvious mistake :)

* Re-work again.

* Fix test.

* cargo +nightly fmt --all

* Make VirtualTip dependent on the transaction size.

* cargo +nightly fmt --all

* Update frame/transaction-payment/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Fix compilation.

* Update bin/node/runtime/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>