Now that all aggregate functions are derived from
stateless_aggregate_function_adapter, we can just fold its functionality
into the base class. This exposes stateless_aggregate_function to
all users of aggregate_function, so they can begin to benefit from
the transformation, though this patch doesn't touch those users.
The aggregate_function base class is partiallly devirtualized since
there is just a single implementation now.
Currently, aggregate functions are implemented in a statefull manner.
The accumulator is stored internally in an aggregate_function::aggregate,
requiring each query to instantiate new instances (see
aggregate_function_selector's constructor, and note how it's called
from selector::new_instance()).
This makes aggregates hard to use in expressions, since expressions
are stateless (with state only provided to evaluate()). To facilitate
migration towards stateless expressions, we define a
stateless_aggregate_function (modelled after user-defined aggregates,
which are already stateless). This new struct defines the aggregate
in terms of three scalar functions: one to aggregate a new input into
an accumulator (provided in the first parameter), one to finalize an
accumulator into a result, and one to reduce two accumulators for
parallelized aggregation.
An adapter of the new struct to the aggregate_function interface is
also provided, to allow for incremental migration in the following
patches.
