add write_pause API

so that we can pause / buffer / resume writes for possible
performance improvements

src/lib/bio/mem.c

@@ -942,3 +942,22 @@ int fr_bio_mem_write_resume(fr_bio_t *bio)
 
 	return my->cb.write_resume(bio);
 }
+
+/** Pause writes.
+ *
+ *  Calls to fr_bio_write() will write to the memory buffer, and not
+ *  to the next bio.  You MUST call fr_bio_mem_write_resume() after
+ *  this to flush any data.
+ */
+int fr_bio_mem_write_pause(fr_bio_t *bio)
+{
+	fr_bio_mem_t *my = talloc_get_type_abort(bio, fr_bio_mem_t);
+
+	if (my->bio.write == fr_bio_mem_write_buffer) return 0;
+
+	if (my->bio.write != fr_bio_mem_write_buffer) return -1;
+
+	my->bio.write = fr_bio_mem_write_buffer;
+
+	return 0;
+}

src/lib/bio/mem.h

@@ -63,3 +63,5 @@ void		fr_bio_mem_read_discard(fr_bio_t *bio, size_t size) CC_HINT(nonnull);
 int		fr_bio_mem_set_verify(fr_bio_t *bio, fr_bio_verify_t verify, void *verify_ctx, bool datagram) CC_HINT(nonnull);
 
 int		fr_bio_mem_write_resume(fr_bio_t *bio) CC_HINT(nonnull);
+
+int		fr_bio_mem_write_pause(fr_bio_t *bio) CC_HINT(nonnull);