Deadlock retries have been accidentally blocked

The Database class has handling in it for deadlocks but some time ago, likely around 4.2, we introduced changes that cause it to be bypassed. When 

$result = $DB->query($string) 

is called & there is a deadlock, it used to return a PEAR_Error object. If the $result was a PEAR_Error then 3 retries were attempted.

However, we made changes to exception handling a while back (around 4.2 I think) that meant that an exception was thrown, by-passing the retry code.

Since this is an unmaintained (& previously hacked) package I think we just patch our version & I propose that we also add a Constant to allow sites to specify the number of retries. This is pretty low-level developer stuff so I don't propose a setting, although some helper text can go in civicrm.settings.php on new installs.

I'm also somewhat inclined to allow some control over the retry period - although probably a second is fine to be a compromise between totally hammering & lingering. To get a longer period retries could be fairly high since a second apart is not crazy.

Dave Jenkins Seamus Lee Jitendra Purohit Monish Deb John Kingsnorth - you may have an interest in this.