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>

substrate/Cargo.lock

@@ -2473,9 +2473,9 @@ dependencies = [
 
 [[package]]
 name = "git2"
-version = "0.13.21"
+version = "0.13.22"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "659cd14835e75b64d9dba5b660463506763cf0aa6cb640aeeb0e98d841093490"
+checksum = "9c1cbbfc9a1996c6af82c2b4caf828d2c653af4fcdbb0e5674cc966eee5a4197"
 dependencies = [
  "bitflags",
  "libc",
@@ -3294,9 +3294,9 @@ checksum = "789da6d93f1b866ffe175afc5322a4d76c038605a1c3319bb57b06967ca98a36"
 
 [[package]]
 name = "libgit2-sys"
-version = "0.12.22+1.1.0"
+version = "0.12.23+1.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "89c53ac117c44f7042ad8d8f5681378dfbc6010e49ec2c0d1f11dfedc7a4a1c3"
+checksum = "29730a445bae719db3107078b46808cc45a5b7a6bae3f31272923af969453356"
 dependencies = [
  "cc",
  "libc",