This replaces the use of a single large ring buffer.
The buffers are still processed in parallel and are recycled to reduce
allocation. Events that do not fit in the default sized buffer have a separate
buffer created to fit them. These large buffers are not recycled.
Separate shared memory is used for readback, with a single shmem cached for this
purpose. Generally only one cached shmem should be required, because the
operations that usually readback the data do it straight away.
Differential Revision: https://phabricator.services.mozilla.com/D193207
818e1b42b7