srv_last_monitor_time: make all accesses relaxed atomical

WARNING: ThreadSanitizer: data race (pid=12041)
  Write of size 8 at 0x000003949278 by thread T26 (mutexes: write M226445748578513120):
    #0 thd_destructor_proxy storage/innobase/handler/ha_innodb.cc:314:14 (mysqld+0x19b5505)

  Previous read of size 8 at 0x000003949278 by main thread:
    #0 innobase_init(void*) storage/innobase/handler/ha_innodb.cc:4180:11 (mysqld+0x1a03404)
    #1 ha_initialize_handlerton(st_plugin_int*) sql/handler.cc:522:31 (mysqld+0xc5ec73)
    #2 plugin_initialize(st_mem_root*, st_plugin_int*, int*, char**, bool) sql/sql_plugin.cc:1447:9 (mysqld+0x134908d)
    #3 plugin_init(int*, char**, int) sql/sql_plugin.cc:1729:15 (mysqld+0x13484f0)
    #4 init_server_components() sql/mysqld.cc:5345:7 (mysqld+0xbf720f)
    #5 mysqld_main(int, char**) sql/mysqld.cc:5940:7 (mysqld+0xbf107d)
    #6 main sql/main.cc:25:10 (mysqld+0xbe971b)

  Location is global 'srv_running' of size 8 at 0x000003949278 (mysqld+0x000003949278)

WARNING: ThreadSanitizer: data race (pid=27869)
  Atomic write of size 4 at 0x7b4800000c00 by thread T8:
    #0 __tsan_atomic32_exchange llvm/projects/compiler-rt/lib/tsan/rtl/tsan_interface_atomic.cc:589 (mysqld+0xbd4eac)
    #1 TTASEventMutex<GenericPolicy>::exit() storage/innobase/include/ib0mutex.h:467:7 (mysqld+0x1a8d4cb)
    #2 PolicyMutex<TTASEventMutex<GenericPolicy> >::exit() storage/innobase/include/ib0mutex.h:609:10 (mysqld+0x1a7839e)
    #3 fil_validate() storage/innobase/fil/fil0fil.cc:5535:2 (mysqld+0x1abd913)
    #4 fil_validate_skip() storage/innobase/fil/fil0fil.cc:204:9 (mysqld+0x1aba601)
    #5 fil_aio_wait(unsigned long) storage/innobase/fil/fil0fil.cc:5296:2 (mysqld+0x1abbae6)
    #6 io_handler_thread storage/innobase/srv/srv0start.cc:340:3 (mysqld+0x21abe1e)

  Previous read of size 4 at 0x7b4800000c00 by main thread (mutexes: write M1273, write M1271):
    #0 TTASEventMutex<GenericPolicy>::state() const storage/innobase/include/ib0mutex.h:530:10 (mysqld+0x21c66e2)
    #1 sync_array_detect_deadlock(sync_array_t*, sync_cell_t*, sync_cell_t*, unsigned long) storage/innobase/sync/sync0arr.cc:746:14 (mysqld+0x21c1c7a)
    #2 sync_array_wait_event(sync_array_t*, sync_cell_t*&) storage/innobase/sync/sync0arr.cc:465:6 (mysqld+0x21c1708)
    #3 TTASEventMutex<GenericPolicy>::enter(unsigned int, unsigned int, char const*, unsigned int) storage/innobase/include/ib0mutex.h:516:6 (mysqld+0x1a8c206)
    #4 PolicyMutex<TTASEventMutex<GenericPolicy> >::enter(unsigned int, unsigned int, char const*, unsigned int) storage/innobase/include/ib0mutex.h:635:10 (mysqld+0x1a782c3)
    #5 fil_mutex_enter_and_prepare_for_io(unsigned long) storage/innobase/fil/fil0fil.cc:1131:3 (mysqld+0x1a9a92e)
    #6 fil_io(IORequest const&, bool, page_id_t const&, page_size_t const&, unsigned long, unsigned long, void*, void*, bool) storage/innobase/fil/fil0fil.cc:5082:2 (mysqld+0x1ab8de2)
    #7 buf_flush_write_block_low(buf_page_t*, buf_flush_t, bool) storage/innobase/buf/buf0flu.cc:1112:3 (mysqld+0x1cb970a)
    #8 buf_flush_page(buf_pool_t*, buf_page_t*, buf_flush_t, bool) storage/innobase/buf/buf0flu.cc:1270:3 (mysqld+0x1cb7d70)
    #9 buf_flush_try_neighbors(page_id_t const&, buf_flush_t, unsigned long, unsigned long) storage/innobase/buf/buf0flu.cc:1493:9 (mysqld+0x1cc9674)
    #10 buf_flush_page_and_try_neighbors(buf_page_t*, buf_flush_t, unsigned long, unsigned long*) storage/innobase/buf/buf0flu.cc:1565:13 (mysqld+0x1cbadf3)
    #11 buf_do_flush_list_batch(buf_pool_t*, unsigned long, unsigned long) storage/innobase/buf/buf0flu.cc:1825:3 (mysqld+0x1cbbcb8)
    #12 buf_flush_batch(buf_pool_t*, buf_flush_t, unsigned long, unsigned long, flush_counters_t*) storage/innobase/buf/buf0flu.cc:1895:16 (mysqld+0x1cbb459)
    #13 buf_flush_do_batch(buf_pool_t*, buf_flush_t, unsigned long, unsigned long, flush_counters_t*) storage/innobase/buf/buf0flu.cc:2065:2 (mysqld+0x1cbcfe1)
    #14 buf_flush_lists(unsigned long, unsigned long, unsigned long*) storage/innobase/buf/buf0flu.cc:2167:8 (mysqld+0x1cbd5a3)
    #15 log_preflush_pool_modified_pages(unsigned long) storage/innobase/log/log0log.cc:1400:13 (mysqld+0x1eefc3b)
    #16 log_make_checkpoint_at(unsigned long, bool) storage/innobase/log/log0log.cc:1751:10 (mysqld+0x1eefb16)
    #17 buf_dblwr_create() storage/innobase/buf/buf0dblwr.cc:335:2 (mysqld+0x1cd2141)
    #18 innobase_start_or_create_for_mysql() storage/innobase/srv/srv0start.cc:2539:10 (mysqld+0x21b4d8e)
    #19 innobase_init(void*) storage/innobase/handler/ha_innodb.cc:4193:8 (mysqld+0x1a5e3d7)
    #20 ha_initialize_handlerton(st_plugin_int*) sql/handler.cc:522:31 (mysqld+0xc74d33)
    #21 plugin_initialize(st_mem_root*, st_plugin_int*, int*, char**, bool) sql/sql_plugin.cc:1447:9 (mysqld+0x1376d5d)
    #22 plugin_init(int*, char**, int) sql/sql_plugin.cc:1729:15 (mysqld+0x13761c0)
    #23 init_server_components() sql/mysqld.cc:5348:7 (mysqld+0xc0d0ff)
    #24 mysqld_main(int, char**) sql/mysqld.cc:5943:7 (mysqld+0xc06f9d)
    #25 main sql/main.cc:25:10 (mysqld+0xbff71b)

WARNING: ThreadSanitizer: data race (pid=29031)
  Write of size 8 at 0x0000039e48e0 by thread T15:
    #0 srv_monitor_thread storage/innobase/srv/srv0srv.cc:1699:24 (mysqld+0x21a254e)

  Previous write of size 8 at 0x0000039e48e0 by thread T14:
    #0 srv_refresh_innodb_monitor_stats() storage/innobase/srv/srv0srv.cc:1165:24 (mysqld+0x21a3124)
    #1 srv_error_monitor_thread storage/innobase/srv/srv0srv.cc:1836:3 (mysqld+0x21a2d40)

  Location is global 'srv_last_monitor_time' of size 8 at 0x0000039e48e0 (mysqld+0x0000039e48e0)