Merge bitcoin/bitcoin#33679: test: set number of RPC server threads to 2

e9cd45e3d3c7592265ebf67387090b3df1501df4 test: set number of RPC server threads to 2 (furszy)

Pull request description:

  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.

ACKs for top commit:
  maflcko:
    lgtm ACK e9cd45e3d3c7592265ebf67387090b3df1501df4
  l0rinc:
    ACK e9cd45e3d3c7592265ebf67387090b3df1501df4
  kevkevinpal:
    ACK [e9cd45e](e9cd45e3d3)
  andrewtoth:
    ACK e9cd45e3d3c7592265ebf67387090b3df1501df4

Tree-SHA512: a777286f4a890fb87f5df72cd2ccfdc628657206a4b3e995044e5a0d12987b8c78a7cf7d684cc4e92605aa782aaeebc44e9f754752c3a524152fac94fa30f4b5

test/functional/test_framework/util.py

@@ -538,6 +538,7 @@ def write_config(config_path, *, n, chain, extra_config="", disable_autoconnect=
         # Disable server-side timeouts to avoid intermittent issues
         f.write("rpcservertimeout=99000\n")
         f.write("rpcdoccheck=1\n")
+        f.write("rpcthreads=2\n")
         f.write("fallbackfee=0.0002\n")
         f.write("server=1\n")
         f.write("keypool=1\n")