Purify get functions

[dohaki]

Most of the getter functions in the library are not pure. They depend on which user is loaded in the state and restrict the functionality of the relay REST API.

Refactor getter functions to be pure, i.e.

// not pure
tlNetwork.trustline.get(network, counterpartyAddress)

// pure
tlNetwork.trustline.get(network, user, counterpartyAddress)

[berndbohmeier]

But the clientlib is not just a lib for easy querying the relay server. We should only make that change if it is not harder for the normal user to use the lib.

[dohaki]

The user of the clientlib would have to additionally pass in one more parameter in the getter functions. Would you consider this as harder to use?
@jensneuber what is your opinion on this?

[andfletcher]

Needs to be handled within a bigger issue that we need to create to handle refactoring of the client lib. If that issue is created this issue can be closed.