fix(ParasService): adjust endpoint to use historicApi, fix endingOffset bug

[TarikGul]

rel: #698

This PR refactors the the ParasService to use a historicApi for querying the chain.

Bug Fix

There was a bug for /experimental/paras/auctions/current?at=8400000 where the incorrect endingOffset was being calculated and passed into auctions.winning which would return the following:

RPC-CORE: getStorage(key: StorageKey, at?: BlockHash): StorageData:: Unable to decode storage auctions.winning:: createType(WinningData):: [Option<(AccountId,ParaId,BalanceOf)>;10]:: Decoded input doesn't match input, received 0x00000000000000016d6f646c70792f6366756e64d10700000000000000000000…0000000000000000000000000000000000000000000000000000000000000000 (88 bytes), created 0x00000000000000016d6f646c70792f6366756e64d107000000000000000000000000000000000000d10700002550b60924ae410100000000000000000000 (62 bytes)
Error: Unable to decode storage auctions.winning:: createType(WinningData):: [Option<(AccountId,ParaId,BalanceOf)>;10]:: Decoded input doesn't match input, received 0x00000000000000016d6f646c70792f6366756e64d10700000000000000000000…0000000000000000000000000000000000000000000000000000000000000000 (88 bytes), created 0x00000000000000016d6f646c70792f6366756e64d107000000000000000000000000000000000000d10700002550b60924ae410100000000000000000000 (62 bytes)

The endingOffset for block 8400000 should be 1478008, but we were receiving 412.

I used the implementation inside of @polkadot/apps in order to fix the current bug.

It also takes into account an edgecase where if the current now block substracted by the offset is less than zero than the leasePeriodIndex will be null.

[emostov]

We actually need to adjust this due to this PR: paritytech/polkadot#3980

So I think it needs to be (now - lease_offset) / lease_period

[TarikGul]

Ahhh okay sweet, thanks for the link. If I am seeing this correctly it is coming from this https://github.com/paritytech/polkadot/blob/master/runtime/common/src/crowdloan.rs#L1015

[TarikGul]

@emostov updated if. Could you take a look when you get a chance

[jsdw]

Looks sane; I'm happy if others are! :)

[emostov]

This needs to be updated slightly. There is an edge case where offset > now and we decided to consider this has have no lease period index. In the substrate code this is represented by None. Here is the line paritytech/polkadot@3668966#diff-3e19d8a242311f202d80bcf9998ffdc22ddd933ed340f308276f353f137e2304R444

So basically it says "if now - offset is less than 0, return None". So we need a update for the logic that uses this function to basically say there is no lease period at now and we could add the first lease period start at block offset.

(This logic which got added to the substrate PR a bit late).

[emostov]

(You can see the changes for this in substrate commit paritytech/polkadot@3668966)

[TarikGul]

Not sure what this should be returning.

[emostov]

I think it should be null