* use channels to implement basic import queue

* async justification import

* better conditional for is_done in tests

* reword the test for presence of link

* fix conditional

* trace instead of panic when no link present

* reword expectations when sending to importers

* fix

* debug justification import error

* update expectations

* use NumberFor

* nits

* add general description

* move error handling into closure

* Encode count of exported blocks correctly

There was a type mismatch: import used u32, export
used Number.

* Wait for import to finish

The issue was that even though the import thread
was still running, the main thread exited.

* Remove superfluous parentheses

* Improve structure, add proofs for expects

* Unify types for export/import length

* wip

* Split bytes fee charging and charging by amount into different traits.

* Move to edition 2018.

* Implemented charge fee traits for fees module.

* Implemented 'on_finalise' for fee module.

* Updated fees finalize impl.

* Renaming and documentation update.

* Added overflow & underflow check for fee calculation.

* Added mock and unit tests for fee module.

* More unit tests for fees module.

* Fixed srml-executive unit tests.

* Remove transaction base/bytes fee from balances module, fix unit tests.

* fix compile error

* Fixed unit test.

* Minor fixes.

* Bump spec version.

* Bump spec version.

* Updated fees module and runtime wasm.

* Fees module code style improvement; updated runtime wasm.

* Bump spec and impl version.

* use DB backend in test client

* Update core/client/db/src/lib.rs

Co-Authored-By: svyatonik <[email protected]>

Ensure the config directory exists before trying to write the public key file.
It is the responsibility of obtain_private_key to do so, as per the purpose
of this function.

* Adding vesting tests to Balances module

* Fix gitlab message, bump impl

* added demo module to node template

* updated comments

* replaced demo with updated template module

* Personal preferences here

* fixed comment inside macro

* core: grandpa: collect garbage for topic

* core: grandpa: use collect_garbage_for_topic in tests