applier: remove writer_cond

Writer condition variable was used by the writer fiber to be woken
up when it is time to send a heartbeat or an ACK.

However it is not really needed, because writer fiber pointer is
always available in the same structure as writer_cond, and can be
used to call fiber_wakeup() directly.

Note, fiber_cond_signal() is basically the same fiber_wakeup().

The patch is not just refactoring for nothing. It is a
prerequisite for #5100. In this issue it will be needed to wakeup
the applier's writer fiber directly on each WAL write from txn.c
module. So the writer_cond won't be available. The only usable
thing will be txn->fiber, which will be set to applier's writer.

Part of #5100

src/box/applier.cc

@@ -155,11 +155,9 @@ applier_writer_f(va_list ap)
 		 * replication_timeout seconds any more.
 		 */
 		if (applier->version_id >= version_id(1, 7, 7))
-			fiber_cond_wait_timeout(&applier->writer_cond,
-						TIMEOUT_INFINITY);
+			fiber_yield_timeout(TIMEOUT_INFINITY);
 		else
-			fiber_cond_wait_timeout(&applier->writer_cond,
-						replication_timeout);
+			fiber_yield_timeout(replication_timeout);
 		/*
 		 * A writer fiber is going to be awaken after a commit or
 		 * a heartbeat message. So this is an appropriate place to
@@ -928,7 +926,7 @@ applier_on_commit(struct trigger *trigger, void *event)
 {
 	(void) event;
 	struct applier *applier = (struct applier *)trigger->data;
-	fiber_cond_signal(&applier->writer_cond);
+	fiber_wakeup(applier->writer);
 	return 0;
 }
 
@@ -1093,7 +1091,7 @@ applier_subscribe(struct applier *applier)
 		 */
 		if (stailq_first_entry(&rows, struct applier_tx_row,
 				       next)->row.lsn == 0)
-			fiber_cond_signal(&applier->writer_cond);
+			fiber_wakeup(applier->writer);
 		else if (applier_apply_tx(&rows) != 0)
 			diag_raise();
 
@@ -1289,7 +1287,6 @@ applier_new(const char *uri)
 	applier->last_row_time = ev_monotonic_now(loop());
 	rlist_create(&applier->on_state);
 	fiber_cond_create(&applier->resume_cond);
-	fiber_cond_create(&applier->writer_cond);
 	diag_create(&applier->diag);
 
 	return applier;

src/box/applier.h

@@ -78,8 +78,6 @@ struct applier {
 	struct fiber *reader;
 	/** Background fiber to reply with vclock */
 	struct fiber *writer;
-	/** Writer cond. */
-	struct fiber_cond writer_cond;
 	/** Finite-state machine */
 	enum applier_state state;
 	/** Local time of this replica when the last row has been received */