Cumulative fixes to make working with consensus-pow easier (#3617)

* consensus-pow: add difficulty data to auxiliary

* Timestamp api

* Implement FinalityProofProvider for ()

* Add DifficultyApi

* Remove assumption that Difficulty is u128

* Use a separate trait for add instead of hard-code it as Saturating

* Some convenience functions to work with PowVerifier

* Try to fix mining unstability

* Fix generic resolution

* Unused best_header variable

* Fix hash calculation

* Remove artificial sleep

* Tweak proposer waiting time

* Revert sleep removal

The reason why it was there is because when mine_loop returns, it means an error
happened. In that case, we'd better sleep for a moment before trying again,
because immediately trying would most likely just fail.

* Pass sync oracle to mining

So that it does not mine when major syncing

* Expose build time as a parameter

Instead of hardcode it as previously 100ms.

* Update lock file

* Fix compile

* Support skipping check_inherents for ancient blocks

For PoW, older blocks are secured by the work, and can mostly be considered to
be finalized. Thus we can save both code complexity and validation time by
skipping checking inherents for them.

* Move difficulty fetch function out of loop

To make things faster

* Remove seed from mining

Each engine can use its own Rng source.

* Better comments

* Add TotalDifficulty definition for U256 and u128

* Update core/consensus/pow/src/lib.rs

Co-Authored-By: André Silva <[email protected]>

* Rename TotalDifficulty::add -> increment

* Use SelectChain to fetch the best header/hash

* Update lock file