Backports to v0.6 (#493)

* Fix compilation in wasm (#465)

Also fix the weird file structure by making `wasm_executor.rs` ->
`wasm_executor/mod.rs`.

* Changes for substrate#3699 (#466)

* change upstream and branch

* Fix build

* remove doc renamings

* Fix tests

* .

* Revert changing fork and branch

* Update Cargo.toml

* Update parachains.rs

* wasm_executor: fix wasm signature checker (#471)

Signed-off-by: yjhmelody <[email protected]>

* Update to latest Substrate master (#472)

* Update to latest Substrate master

* Fix

* Fix compilation

* fix var name for post_upward_message (#474)

Signed-off-by: yjhmelody <[email protected]>

* integrate minor weight/fee changes (#482)

* Update cargo files

* Make it build again.

* Fix build

* revert cargo file

* New lockfile

* Bump.

* Update to latest Substrate master (#486)

* Only register one gossip validator for full nodes (#487)

* Support `account_nextIndex` RPC. (#460)

* Use node-rpc extensions to support account_nextIndex.

* Remove todo.

* Update lock.

* Use new srml_system_rpc crate.

* Update to substrate=master

* Update lockfile.

* Update to polkadot-master.

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <[email protected]>

* Update to latest substrate master (#491)

* update to latest substrate master

* Fix compilation