Limit minium aria_block_size to 4096

MDEV-19585  Assertion with S3 table and flush_tables

The limit has to be increased so that MariaDB can create system tables.
It should not have any notable impact on performance.

There should not be any notable performance differences between 1K and 4K,
especially for temporary tables. In most cases using bigger blocks is also
faster (with the possible exception of doing key reads of not fixed length
keys).

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

@@ -9,7 +9,7 @@ DEFAULT_VALUE	8192
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	BIGINT UNSIGNED
 VARIABLE_COMMENT	Block size to be used for Aria index pages.
-NUMERIC_MIN_VALUE	1024
+NUMERIC_MIN_VALUE	4096
 NUMERIC_MAX_VALUE	32768
 NUMERIC_BLOCK_SIZE	1024
 ENUM_VALUE_LIST	NULL

storage/maria/ha_maria.cc

@@ -152,10 +152,11 @@ static void update_log_file_size(MYSQL_THD thd,
                                  struct st_mysql_sys_var *var,
                                  void *var_ptr, const void *save);
 
+/* The 4096 is there because of MariaDB privilege tables */
 static MYSQL_SYSVAR_ULONG(block_size, maria_block_size,
        PLUGIN_VAR_RQCMDARG | PLUGIN_VAR_READONLY,
        "Block size to be used for Aria index pages.", 0, 0,
-       MARIA_KEY_BLOCK_LENGTH, MARIA_MIN_KEY_BLOCK_LENGTH,
+       MARIA_KEY_BLOCK_LENGTH, 4096,
        MARIA_MAX_KEY_BLOCK_LENGTH, MARIA_MIN_KEY_BLOCK_LENGTH);
 
 static MYSQL_SYSVAR_ULONG(checkpoint_interval, checkpoint_interval,