Sqlite::truncate(): Verify `sqlite_sequence` exists before _execute()

`sqlite_sequence` is a dynamic table that's only available when a table in
the database use an auto increment field.  For some cases, eg: databases that
exclusively use uuid for primary keys, this table won't exist and
truncate() call will fail with:

  Error: SQLSTATE[HY000]: General error: 1 no such table: sqlite_sequence
Reduce variable use/duplicate code.
Added/removed newlines to make the code more readable.
flushMethodCache not working if called before cacheMethod

When calling flushMethodCache it correctly empties self::$methodCache but if cacheMethod is called afterwards self::$methodCache gets populated with data from the Cache.
performance improvements in Model::_clearCache

- don't empty cache twice if pluralized name is identical to underscored
- refactors for fewer function calls
Refactors Model::_clearCache

- don't use mixed keys in $assoc array
- removes unnecessary strtolower on Inflector::underscore result
- return early
Test case for DboSource::flushMethodCache
Fixing CS errors.
Updated returns in docblocks in File utility
Fixed CS error
Additional grammatical corrections to File utility
Rrue to True, simple typo
Merge pull request #1701 from bcrowe/hotfix-phpcs

Fixed CS error
Merge pull request #1700 from bcrowe/hotfix-filedocblocks

Updated returns in docblocks in File utility
Fixed setting "required" attribute for file input. Closes #4124

In general fixed the issue where enabling SECURE_SKIP for a field
skipped "required" attribute check altogether. Instead now "required"
is set to false for hidden fields by default.
Merge pull request #1703 from ADmad/2.4-file-input-required

Fixed setting "required" attribute for file input. 

Fixes #4124
Merge pull request #1687 from Schlaefer/model-cache-improvements

performance improvements in Model::_clearCache
Correct spelling mistake in class name.
fixes FileLog doesn't delete on rotation if count(files) is greater 'rotate'
fixes FileLog doesn't delete on rotation if `rotate` = 0