Allow using a single mem object read-only across multiple devices in parallel

Right now a mem object has a single active device and the contents are always migrated when it's used somewhere else. Instead, we should track multiple active devices, and only invalidate others when a (potentially) writing operation is done. Read-only maps, copies as a source, binds to constant memory, or binds to read-only images should all avoid invalidating other device's contents.

Start by locating the mem object implementation and the paths that migrate or invalidate device contents; no files or tests are named in the issue. Trace read-only maps, source copies, constant-memory binds, read-only image binds, and writing operations, then verify that read-only uses can remain parallel while writes invalidate other devices.