Merge seastar upstream

* seastar a14d667...54e940f (8):
  > Merge "Prometheus to use output stream" from Amnon
  > http_test: Fix an http output stream test
  > build: harden try_compile_and_link output temporary file
  > configure: disable exception scalability hack on debug build
  > build: don't perform test compiles to /dev/null
  > Provide workaround for non scaleable c++ exception runtime
  > Merge "Add output stream to http message reply" from Amnon
  > configure.py: use user provided compiler flags when checking for features