lib: move handling of data->req.writer_stack into Curl_client_write()

- move definitions from content_encoding.h to sendf.h
- move create/cleanup/add code into sendf.c
- installed content_encoding writers will always be called
  on Curl_client_write(CLIENTWRITE_BODY)
- Curl_client_cleanup() frees writers and tempbuffers from
  paused transfers, irregardless of protocol

Closes #11908

lib/multi.c

@@ -665,7 +665,6 @@ static CURLcode multi_done(struct Curl_easy *data,
 {
   CURLcode result;
   struct connectdata *conn = data->conn;
-  unsigned int i;
 
 #if defined(DEBUGBUILD) && !defined(CURL_DISABLE_VERBOSE_STRINGS)
   DEBUGF(infof(data, "multi_done[%s]: status: %d prem: %d done: %d",
@@ -721,12 +720,7 @@ static CURLcode multi_done(struct Curl_easy *data,
 
   Curl_safefree(data->state.ulbuf);
 
-  /* if the transfer was completed in a paused state there can be buffered
-     data left to free */
-  for(i = 0; i < data->state.tempcount; i++) {
-    Curl_dyn_free(&data->state.tempwrite[i].b);
-  }
-  data->state.tempcount = 0;
+  Curl_client_cleanup(data);
 
   CONNCACHE_LOCK(data);
   Curl_detach_connection(data);