Fix data race in asio service

[antonio2368]

We detected a data race with one of our tests
https://pastila.nl/?0003c12b/68530aca8742766996176fdebcede8dc#KBCq01QCyL4Qz2lDbsNzrA==

It's insanely hard to parse all that asio stacktrace but the race happens on operation_timer_ inside async_resolve callback.
My theory is that async_connect got resolved and had its callback called before we setup the timer and started it.
So the problem should be solved with moving timer setup before we start the async_connect.

Now for the general issue.
Asio in NuRaft is run with a single io context on multiple threads. This can lead to data races in general (e.g. timer calls socket.cancel()). I'm not sure if cancel on socket is threadsafe (I assume it's not) but to avoid similar problems shouldn't strand be used for a single asio_rpc_client?
If you think I misunderstood something or there is a simpler solution, I'm open for discussion.
If not, I can introduce strand for the client.