message: Improve reliablility of sendheaders.py
  This script relies a great deal on sync_with_ping(), as a result we
  need to make the client's single threaded in regards to net and message
  processing.  Only allow 1 txAdmissionThread and 1 msgHandlerThread
diff: @@ -286,7 +286,7 @@ class SendHeadersTest(BitcoinTestFramework):
  # Currently there are mininode syncronization issues when Parallel Validation is turned on
  # and therefore have -parallel=0 when running these tests.
  self.nodes = []
  - args = [["-debug", "-logtimemicros=1", "-parallel=0", "-use-thinblocks=0", "-use-grapheneblocks=0", "-use-compactblocks=0"]]*2
  + args = [["-debug", "-logtimemicros=1", "-parallel=0", "-use-thinblocks=0", "-use-grapheneblocks=0", "-use-compactblocks=0", "-net.msgHandlerThreads=1", "-net.txAdmissionThreads=1"]]*2
  self.nodes = start_nodes(2, self.options.tmpdir, args)
  connect_nodes(self.nodes[0], 1)
  