Rework memory management to allow shared memory data transfer. The central idea

Rework memory management to allow shared memory data transfer. The central idea
is to allocate all audio memory blocks from a per-process memory pool which is
available as read-only SHM segment to other local processes. Then, instead of
writing the actual audio data to the socket just write references to this
shared memory pool.

To work optimally all memory blocks should now be of type PA_MEMBLOCK_POOL or
PA_MEMBLOCK_POOL_EXTERNAL. The function pa_memblock_new() now generates memory
blocks of this type by default.

git-svn-id: svn://svn.0pointer.net/pulseaudio/trunk@1266 fefdeb5f-60dc-0310-8127-8f9354f1896f