rpcserver: Decouple agenda info status strings.
rpcserver: Decouple agenda info status strings.

Currently, the getblockchainfo RPC implementation directly returns the
internal threshold state strings defined in blockchain.  This is not
ideal since it means that any changes to the internal implementation
could inadvertently change the RPC results without being noticed.

It also means that consumers of the RPC have to import the blockchain
module solely to access those constants.  Not only does that mean an
extremely heavy dependency is required just to access those constants,
the module is eventually going to be removed in favor of making the
blockchain package an internal package meaning clients will no longer
have access to it.

Consequently, this updates the getblockchaininfo RPC implementation to
decouple the agenda status strings by converting the internal blockchain
threshold state to use the new agenda status string constants defined in
the rpc/jsonrpc/types module.

decred/dcrd,decred/dcrd