Implement serf_request_read_iovec to further condense TCP writev() calls.
Implement serf_request_read_iovec to further condense TCP writev() calls.

* buckets/request_buckets.c
  (serf_request_read_iovec): Implement.
  (serf_bucket_type_request): Delegate to the aggregate bucket directly
  instead of letting a level of indirection separate the read() passes.

jandre/serf,jandre/serf,jandre/serf,jandre/serf