Session bug: JDatabaseMySQL Duplicate entry - vendor neutral fix

This is the vendor neutral fix, I have another pull request open with a preferred mysql specific fix.

## Abstract

A bug exists in JSessionStorageDatabase::write() (libraries/joomla/session/storage/database.php:94).  The logic assumes that an update will have one affected row if a session exists.  However, this is not true.  The timestamp is only granular in seconds and may be the same with multiple ajax requests, refreshes on different tabs, etc.  When the timestamp is the same the entry is exactly the same and the update will not affect any rows.  With the current logic an insert will be attempted and this results in a 500.

## The fix

This fix uses a SELECT COUNT to determine a session row currently exists and should be updated.


## Example Error

<pre>
500 - JDatabaseMySQLi::query: 1062 - Duplicate entry 'e33c3ab2b05a9dc97dc25187a823b8e6' for key 'PRIMARY' SQL=INSERT INTO `jos_session` (`session_id`, `data`, `time`) VALUES ('e33c3ab2b05a9dc97dc25187a823b8e6', '__default|a:8:{s:22:\"session.client.browser\";s:119:\"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.54 Safari/535.19\";s:15:\"session.counter\";i:43;s:8:\"registry\";O:9:\"JRegistry\":1:{s:7:\"\0*\0data\";O:8:\"stdClass\":0:{}}s:4:\"user\";O:5:\"JUser\":23:{s:9:\"\0*\0isRoot\";b:0;s:2:\"id\";i:0;s:4:\"name\";N;s:8:\"username\";N;s:5:\"email\";N;s:8:\"password\";N;s:14:\"password_clear\";s:0:\"\";s:8:\"usertype\";N;s:5:\"block\";N;s:9:\"sendEmail\";i:0;s:12:\"registerDate\";N;s:13:\"lastvisitDate\";N;s:10:\"activation\";N;s:6:\"params\";N;s:6:\"groups\";a:0:{}s:5:\"guest\";i:1;s:10:\"\0*\0_params\";O:9:\"JRegistry\":1:{s:7:\"\0*\0data\";O:8:\"stdClass\":0:{}}s:14:\"\0*\0_authGroups\";a:1:{i:0;i:1;}s:14:\"\0*\0_authLevels\";a:2:{i:0;i:1;i:1;i:1;}s:15:\"\0*\0_authActions\";N;s:12:\"\0*\0_errorMsg\";N;s:10:\"\0*\0_errors\";a:0:{}s:3:\"aid\";i:0;}s:13:\"session.token\";s:32:\"cc3f66462deb72a899115b2ea51f8608\";s:19:\"session.timer.start\";i:1331743210;s:18:\"session.timer.last\";i:1331744856;s:17:\"session.timer.now\";i:1331744857;}', '1331744857')
</pre>