Slowdowns and timeout with concurrent connections

[darosior]

We use Tokio. However, most (all?) of the IO we are doing is blocking (see revault/revault_net#22).
I believe this is the cause for some pretty bad performances noticed of the coordinator under reasonable load. For instance, we would regularly encounter connection timeouts when testing on Signet and even more so when running revaultd's functional test suite. I believe fixing this issue would greatly reduce the runtime of the functional tests on revaultd.

So, here is what we need to do:

1. Confirm (or reject) the hypothesis that the performance loss / timeouts is due to a lack of real asynchronicity. The slowdowns can be reproduced using revaultd's functional test suite.
2. Once we have found the cause, obviously fix it here. If it's effectively due to blocking code we will probably need to fix Add a tokio feature revault_net#22.