WL#14008: InnoDB: Make tablespace filename validation optional via --innodb-validate-tablespace-paths

Affects: Server-8.0   —   Status: Complete

By default we take a conservative/pessimistic approach. On startup we always check 
the path of all known table spaces against the information stored in the data 
dictionary. In case the user has moved files around. This can be very costly when 
the number of tablespace is high e.g., more than 100K.

This WL introduces a new variable --innodb-validate-tablespace-paths := (ON | OFF) 
and the default will be ON to preserve existing behavior. In situations where we 
know that the user cannot move files around e.g. in a managed service we can set 
this variable to OFF. This reduces the startup time significantly.
FR1 - The tablespace path validation can be skipped using --innodb-validate-
tablespace-paths=0 or --skip-innodb-validate-tablespace-paths

FR2 - A [Note] message is logged in case tablespace path validation is skipped.
Introduce a new MySQL variable indicating whether InnoDB should check the 
tablespace paths in the data dictionary against the actual directories. 

  [1] ./bin/mysqld --skip-innodb-validate-tablespace-paths
  [2] ./bin/mysqld --innodb-validate-tablespace-paths=0

In both examples InnoDB will log a Note message stating that the validation is 
    ... [Note] [...] [InnoDB] Skipping InnoDB tablespace path validation. Manually 
moved tablespace files will not be detected!
Since this is a [Note] it is only visible when log error verbosity is 3.

When using this flag, InnoDB will not detect any files that get moved between 
different known directories. This should be used in environments where the user 
cannot move files around. As such, this is a desirable change for MySQLaaS as it 
provides a reduction in startup times.
1) Declare the new MYSQL_SYSVAR_BOOL in ha_innodb.cc - validate_tablespace_paths:

    validate_tablespace_paths, srv_validate_tablespace_paths,
    "Enable validation of tablespace paths against the DD. (default: on)."
    " Disable with --skip-innodb-validate-tablespace-paths.",
    nullptr, nullptr, TRUE);

2) Declare the underlying bool (srv_validate_tablespace_paths) in srv0srv.h and 
define in srv0srv.cc:

  /** Whether to validate InnoDB tablespace paths on startup */
  bool srv_validate_tablespace_paths = true;

4) Introduce new message in messages_to_error_log.txt:
    eng "Skipping InnoDB tablespace path validation. Manually moved tablespace 
files will not be detected!"

3) In ha_innodb.cc, bool boot_tablespaces(THD *thd, size_t *moved_count) add a 
check before calling the validation code:

  if (srv_validate_tablespace_paths) {
    Validate_files validator;
    return (validator.validate(tablespaces, moved_count) != DB_SUCCESS);

  return false;