Create nonblocking version of BlockhashQuery

[2501babe]

Problem

BlockhashQuery is an interface that allows a program to transparently fetch a blockhash from rpc or substitute a hardcoded one depending on what mode its in. the rpc client has been updated to have a nonblocking version, which we are trying to move to in various places (notably for me the token cli), but there isnt a nonblocking version of this yet

Proposed Solution

implement a nonblocking version