[FIX] crm.opportunity.report: query performance

Using a subselect is more efficient than constructing
a complete temporary table of all messages, and then
joining them. This way only messages or relevant
opportunities are used, as most of the time the
report is accessed for a specific subset of opportunities
(by date, salesman, stage, etc.).
The query planner can then use the dedicated index
to quickly access the corresponding messages.

On a sample database with 1 million crm.lead
entries and 10 million mail.message entries
this brings the time for a typical read_group
call on crm.opportunity.report down from 300s to
500ms (600x faster)

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