openquake/engine, tests:

Renamed `CalculatorNext` to `Calculator` and `BaseHazardCalculatorNext` to
`BaseHazardCalculator`.
calcs/hazard/general:

Moved some DB query statements in `task_arg_gen` to separate methods
to make things easier to mock and test.
calcs/hazard/event_based/core:

Updated `task_arg_gen` to split point sources into blocks of the correct size.
tests/calcs/hazard/general_test:

Added a test to exercise the logic of `BaseHazardCalculator.task_arg_gen`.