Client: Refactor CoreContext

[ebuchman]

CoreContext is a little bit of a grab-bag of fields. We should change it to TxContext so it only has fields for signing a tx, ideally only fields that all txs use. Actually, in so far as it builds a StdTx, it might belong in x/auth/client. We can have a separate context for making queries, or maybe just pass in the couple of params as needed, but the TrustNode and Height fields don't belong with the rest of them.

We should also split up SignAndBuild into two separate functions since we intend to support the ability for LCD users to just build the tx, and sign it somewhere else. This should also help inform the CoreContext breakup and the building of better abstractions in the client package.

This is in some sense a follow up of #1157, as some of the improvements discussed there have been merged, but we can keep going

[alexanderbez]

I'll pick this up @ebuchman 👍