bufferpool: Use TAG_MEMORY to check memory before releasing

bufferpool: Use TAG_MEMORY to check memory before releasing

Tag allocated buffers with TAG_MEMORY. When they are released later,
only add them back to the pool if the tag is still there and the memory
has not been changed, otherwise throw the buffer away.
Add unit test to check various scenarios.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=724481