library function for api call "anchors"

[WhoSoup]

closes: #125

Adds two functions to the library: GetAnchors(hash string) and GetAnchorsByHeight(height int64) which cover the available parameters of the "anchors" endpoint: https://docs.factom.com/api#anchors

They return the response in the form of an Anchors struct. I ran into some difficulties as factomd only has ethereum anchors starting at height 202428 and before that it returns "false". Same for Bitcoin anchors in non-mainnet chains.

To avoid having to deal with unmarshalling into an interface{} and then painstakingly transferring the data to a struct, I opted to just find-replace "false" with "null" for the ethereum and bitcoin fields.

[sambarnes]

They return the response in the form of an Anchors struct. I ran into some difficulties as factomd only has ethereum anchors starting at height 202428 and before that it returns "false". Same for Bitcoin anchors in non-mainnet chains.

That is because the original code path that factomd parses anchors is on insertion of entries into the database. So that is the height that you did the upgrade to the ethereum anchor parsing version. But if you restart your node with --reparseanchorchains flag it will re-index all the anchors and give you all ethereum anchors before that height.

I agree it should have been a null instead of a bool there

[WhoSoup]

But if you restart your node with --reparseanchorchains flag it will re-index all the anchors and give you all ethereum anchors before that height.

Ah, interesting. The custom unmarshaller is still necessary due to local chains not having anchors but that's good to know.

[ilzheev]

@carryforward can we merge it into master while preparing the next release?

[PaulBernier]

Can you add a test like you did for #140?

[WhoSoup]

Can you add a test like you did for #140?

unit test added