MDEV-4506: Parallel replication of group-committed transactions: Intermediate commit

First very rough sketch. We spawn and retire a pool of slave threads.
Test main.alias works, most likely not much else does.

sql/rpl_parallel.h

@@ -0,0 +1,74 @@
+#ifndef RPL_PARALLEL_H
+#define RPL_PARALLEL_H
+
+#include "log_event.h"
+
+
+struct rpl_parallel;
+struct rpl_parallel_entry;
+struct rpl_parallel_thread_pool;
+
+class Relay_log_info;
+struct rpl_parallel_thread {
+  bool delay_start;
+  bool running;
+  bool stop;
+  bool free;
+  mysql_mutex_t LOCK_rpl_thread;
+  mysql_cond_t COND_rpl_thread;
+  struct rpl_parallel_thread *next;             /* For free list. */
+  struct rpl_parallel_thread_pool *pool;
+  THD *thd;
+  struct rpl_parallel_entry *current_entry;
+  struct queued_event {
+    queued_event *next;
+    Log_event *ev;
+    Relay_log_info *rli;
+  } *event_queue, *last_in_queue;
+  rpl_parallel_thread *wait_for; /* ToDo: change this ... */
+};
+
+
+struct rpl_parallel_thread_pool {
+  uint32 count;
+  struct rpl_parallel_thread **threads;
+  struct rpl_parallel_thread *free_list;
+  mysql_mutex_t LOCK_rpl_thread_pool;
+  mysql_cond_t COND_rpl_thread_pool;
+  bool changing;
+  bool inited;
+
+  rpl_parallel_thread_pool();
+  int init(uint32 size);
+  void destroy();
+  struct rpl_parallel_thread *get_thread(rpl_parallel_entry *entry);
+};
+
+
+struct rpl_parallel_entry {
+  uint32 domain_id;
+  uint32 last_server_id;
+  uint64 last_seq_no;
+  uint64 last_commit_id;
+  bool active;
+  rpl_parallel_thread *rpl_thread;
+};
+struct rpl_parallel {
+  HASH domain_hash;
+  rpl_parallel_entry *current;
+
+  rpl_parallel();
+  ~rpl_parallel();
+  rpl_parallel_entry *find(uint32 domain_id);
+  bool do_event(Relay_log_info *rli, Log_event *ev, THD *thd);
+};
+
+
+extern struct rpl_parallel_thread_pool global_rpl_thread_pool;
+
+
+extern int rpl_parallel_change_thread_count(rpl_parallel_thread_pool *pool,
+                                            uint32 new_count,
+                                            bool skip_check= false);
+
+#endif  /* RPL_PARALLEL_H */