Always ref writable streams

We suffered a bug whereby doing a follow-up write to another write could
resurrect a deleted stream, causing all sorts of crash.

Fix: when a stream becomes writable (vs when we start writing) take a
ref on the stream, and only relinquish it once we're done writing.
diff --git a/grpc.def b/grpc.def
index bd0bc85..f81aa1b 100644
--- a/grpc.def
+++ b/grpc.def
@@ -182,6 +182,7 @@
     gpr_event_wait
     gpr_ref_init
     gpr_ref
+    gpr_ref_non_zero
     gpr_refn
     gpr_unref
     gpr_stats_init