MDEV-20487 Set innodb_adaptive_hash_index=OFF by default

Based on the performance testing that was conducted in MDEV-17492,
the InnoDB adaptive hash index could only help performance in specific,
almost-read-only workloads. It could slow down all kinds of workloads
(especially DROP TABLE, TRUNCATE TABLE, ALTER TABLE, or DROP INDEX
operations), and it can become corrupted, causing crashes (such as
MDEV-18815, MDEV-20203) and possibly data corruption. Furthermore,
the adaptive hash index consumes space from the InnoDB buffer pool,
which could hurt performance when the working set would almost fit
in the buffer pool.

Given all this, it is best to disable the adaptive hash index by default.

extra/mariabackup/xtrabackup.cc

@@ -1239,11 +1239,10 @@ struct my_option xb_server_options[] =
    GET_STR, OPT_ARG, 0, 0, 0, 0, 0, 0},
 #ifdef BTR_CUR_HASH_ADAPT
   {"innodb_adaptive_hash_index", OPT_INNODB_ADAPTIVE_HASH_INDEX,
-   "Enable InnoDB adaptive hash index (enabled by default).  "
-   "Disable with --skip-innodb-adaptive-hash-index.",
+   "Enable InnoDB adaptive hash index (disabled by default).",
    &btr_search_enabled,
    &btr_search_enabled,
-   0, GET_BOOL, NO_ARG, 1, 0, 0, 0, 0, 0},
+   0, GET_BOOL, NO_ARG, 0, 0, 0, 0, 0, 0},
 #endif /* BTR_CUR_HASH_ADAPT */
   {"innodb_autoextend_increment", OPT_INNODB_AUTOEXTEND_INCREMENT,
    "Data file autoextend increment in megabytes",

mysql-test/suite/sys_vars/r/sysvars_innodb.result

@@ -35,10 +35,10 @@ READ_ONLY	NO
 COMMAND_LINE_ARGUMENT	REQUIRED
 VARIABLE_NAME	INNODB_ADAPTIVE_HASH_INDEX
 SESSION_VALUE	NULL
-DEFAULT_VALUE	ON
+DEFAULT_VALUE	OFF
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	BOOLEAN
-VARIABLE_COMMENT	Enable InnoDB adaptive hash index (enabled by default).  Disable with --skip-innodb-adaptive-hash-index.
+VARIABLE_COMMENT	Enable InnoDB adaptive hash index (disabled by default).
 NUMERIC_MIN_VALUE	NULL
 NUMERIC_MAX_VALUE	NULL
 NUMERIC_BLOCK_SIZE	NULL

storage/innobase/handler/ha_innodb.cc

@@ -18744,9 +18744,8 @@ static MYSQL_SYSVAR_BOOL(stats_traditional, srv_stats_sample_traditional,
 #ifdef BTR_CUR_HASH_ADAPT
 static MYSQL_SYSVAR_BOOL(adaptive_hash_index, btr_search_enabled,
   PLUGIN_VAR_OPCMDARG,
-  "Enable InnoDB adaptive hash index (enabled by default). "
-  " Disable with --skip-innodb-adaptive-hash-index.",
-  NULL, innodb_adaptive_hash_index_update, true);
+  "Enable InnoDB adaptive hash index (disabled by default).",
+  NULL, innodb_adaptive_hash_index_update, false);
 
 /** Number of distinct partitions of AHI.
 Each partition is protected by its own latch and so we have parts number