Share metadata between kv_engine and magma

Currently a (now out of date) copy of the kv metadata is stored in magma in https://github.com/couchbase/magma/blob/master/util/kvengine.h for the purpose of dumping it in magma_dump. It would be ideal to use the kv_engine metadata directly instead (so that we don't have an out of date version lying around that we could forget to update). We should do the same for the UserStats which are also dumped with magma_dump, and perhaps some other structures in the kvengine.h file.