Cache Civi-Report results

This ticket follows on from CRM-17663 where we agreed the caching for CiviCRM dashlets is not optimal and that if we want report results caching it should happen in the reports context somehow.

We had a long discussion on IRC about how it might work - without a firm resolution - the discussion is attached to CRM-17663

To summarise - the key challenge is that if we want to cache less often than per user we need to determine a way to incorporate some representation of the users ACLs into the cache key. We can assume that all contacts with 'view all contacts' can share a cached copy of the report (perhaps making some checks around things like 'view all contributions' as relevant.

Not all reports apply ACLS & those that do generally call buildACLClause - so it may be possible to generate a hash from there. There is some risk reports could do 'something different' and that might need to be fleshed out a bit.

The alternative approach is more like a query level cache - the issue is that many reports generate temp tables - perhaps the tables could be 'kept around for a bit' like export tables, or the results from a sequence of queries could be cached - with the whole sequence comprising the results e.g

$results = Civi::cacheableSql()
->add("SELECT ....")
->add("CREATE TEMP... ")
->add("SELECT ...")
->execute();

I will paste an extract of the longer IRC discussion as a comment