Add FILTER clause for aggregate functions in HQL

[beikov]

Copied over from https://trello.com/c/eyaZ8pcb/204-filter-clause-for-aggregate-functions

Support the SQL standard FILTER clause for aggregate functions in HQL. It can be emulated by doing <<aggregate_function>>(case when <<filter_predicate>> then <<argument>> else null end) because all aggregate functions are null rejecting.

Here some examples:

• SUM(tx.amount) FILTER (WHERE tx.category = 'work') can be emulated as SUM(CASE WHEN tx.category = 'work' THEN tx.amount ELSE NULL END)
• COUNT(*) FILTER (WHERE alias.isActive = true) can be emulated as COUNT(CASE WHEN alias.isActive = true THEN 1 ELSE NULL END)

I needed to add discriminate normal functions from aggregate functions for the lock handling and I introduced as part of that PR the filter predicate on aggregate functions, but I wanted to get everyone's opinion first before also adding the HQL syntax for this.

[beikov]

I'll consider the silence to be equal to agreement on adding support for this.

[dreab8]

I have nothing against it