Add thread_id to JSON of Metadata Blocks

[andrew-lunarg]

Several of the thread_id fields of metadata blocks have comments casting doubt on their contents. E.g.:

...
// thread_id is here as a placeholder. Currently always set to 0.
...
// NOTE: This is currently the ID of the thread that processed the dirty pages and wrote them to
// the file, which may not be the thread that originally modified the memory pages.
...

We currently ignore this field for all metadata blocks but we should output whatever value is stored for each metablock even if it doesn't have much value, at least for the future possibility of doing a near-bit-exact binary->json->binary roundtrip.