mem: improve memory handling

Add a memory pool to manage blocks of memory. Make it possible
to allocate and import blocks.

Add add_mem and remove_mem to the core events to signal a client
of a block of memory. Remove the client-node add_mem.

Make a global pool for memory and a per client pool where we
import and share the memory we need with the client.

Use the new memory pool to track and map memory in clients.

pipewire-jack

@@ -1 +1 @@
-Subproject commit 78109838a96ff94988df9eee1976f99ddcc938b2
+Subproject commit 135b72ce70b5da10424f664edcb531bf530618b1