pulsecore: Transform pa_mempool_new() into a factory method

pulsecore: Transform pa_mempool_new() into a factory method

Soon we're going to have three types of memory pools: POSIX shm_open()
pools, memfd memfd_create() ones, and privately malloc()-ed pools.

Thus introduce annotations for the memory types supported and change
pa_mempool_new() into a factory method based on required memory.

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>