The default -rpcthreads value spawns 16 HTTP server threads for each node.
Running the functional test suite with default rpcthreads can exhaust file
descriptors or hit other resource limits very easily (more when tests are run
in parallel).
Furthermore, having 16 threads is unnecessary since they are mostly idle. We
run RPC calls on a single RPC connection and wait for it result synchronously.
There is (almost) never two RPC calls occurring concurrently.
Because of this, the threads are mostly idle, so we can safely limit the number
of them to two.
Note for reviewers: I checked this does not introduce any timing regression but would be good to double-check it on your end too. We could add another thread if needed. Just the 16 threads default value is too high and unnecessary.
