CRM-16247 Incoming payments can be lost when report 'ties up' activity contact table

I've just hit an issue where IPNs have not been recorded because the activity contact table was being hammered by a search or report at the time it came in.

While obviously there is a problem with the report (not yet identified) I would also thing the payment should be committed to the database prior to the activity being recorded as failure to record the activity does not seem like a good reason not to record the contribution