tests: Expand HTTP coverage to assert libevent behavior

Covers:
- http pipelining
- rpcservertimeout

  Testing this requires adding an option to TestNode to force
  the test framework to establish a new HTTP connection for
  every RPC. Otherwise, attempting to reuse a persistent connection
  would cause framework RPCs during startup and shutdown to fail.

- "chunked" Transfer-Encoding

test/functional/test_framework/test_node.py

@@ -156,6 +156,7 @@ class TestNode():
         self.process = None
         self.rpc_connected = False
         self.rpc = None
+        self.reuse_http_connections = True # Must be set before calling get_rpc_proxy() i.e. before restarting node
         self.url = None
         self.log = logging.getLogger('TestFramework.node%d' % i)
         # Cache perf subprocesses here by their data output filename.
@@ -280,6 +281,7 @@ class TestNode():
                     timeout=self.rpc_timeout // 2,  # Shorter timeout to allow for one retry in case of ETIMEDOUT
                     coveragedir=self.coverage_dir,
                 )
+                rpc.auth_service_proxy_instance.reuse_http_connections = self.reuse_http_connections
                 rpc.getblockcount()
                 # If the call to getblockcount() succeeds then the RPC connection is up
                 if self.version_is_at_least(190000) and wait_for_import: