[Viewer] Fetch users once per page

[fleupold]

We currently fetch the next user "on demand" whenever we exhaust the order for the current one. #641 noticed that the act of getting a single user is querying the size() of the allUser struct und thus actually iterates the list of all users in the system.

Consequently, getting all users is just twice as expensive as getting a single user (once for checking the size and once for getting them) mod the memory of storing them.

For a single unfiltered page we can have at most pageSize users (a user is only in the system if they have at least on order).

This PR therefore replaces the "fetch n times one user when needed" logic with "fetch once n users at the beginning" and then read the next user from that list.

Test Plan

No logic is changed (unit tests keep running). Notice, that now we can query the open orderbook with a page size of 1000 on a standard infura node:

npx truffle console --network mainnet

let instance = await BatchExchangeViewer.at("0xd7a78A333BAe9AAdDD5ebE41C209Fb5226eA155B")
let page = {nextPageUser: "0x0000000000000000000000000000000000000000", nextPageUserOffset:0}
page = await instance.contract.methods.getOpenOrderBookPaginated([], page.nextPageUser, page.nextPageUserOffset, 1000).call()
// ... continue last call until nextPage = false

[fleupold]

Merging early as @fedgiac needs the new package version. If there is an issue with the code we can easily re-deploy.