Fixed naming convention violations.

Summary: This file contains member variables prefixed with underscores. This violates the naming convention enforced by the linter.

Test Plan:
The linter warnings have disappeared. Example:
```
>>> Lint for src/infrastructure/storage/lisk/LiskDAO.php:

   Warning  (XHP9) Naming Conventions
    Follow naming conventions: class properties should be named using
    lowerCamelCase.

             183
             184   const COUNTER_TABLE_NAME          = 'lisk_counter';
             185
    >>>      186   private $__dirtyFields            = array();
             187   private $__missingFields          = array();
             188   private static $processIsolationLevel       = 0;
             189   private static $transactionIsolationLevel   = 0;
```

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8348