wireprotov1peer: simplify the way batchable rpcs are defined
The scheme with futures/generator is confusing due to the way
communication is done by side effects, especially with two different
"future" objects. Just returning a request and a function to read the
response is easier to understand.
There are tests failures with the largefiles extension due to it
aliasing one rpc to another one, which gets fixed in the next commit.
Differential Revision: https://phab.mercurial-scm.org/D11211