Make all IPN scripts to derive contact-id from other info in post params, instead of making it a required field

Other info like contribution-recur-id / invoice-id / contribution-id etc could be useful in deriving the contact-id.

Use Cases :
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1. As mentioned by Brian :- if you merge two contacts, and the trashed one had recurring contributions attached to it, the current contributions will be merged but any future contributions will be attached to the trashed contact.
The reason looks to be related to trashed contact-id being passed by IPN post params.

2. http://forum.civicrm.org/index.php?topic=18750.0