KV callback to read metadata out of snapshot

Magma will invoke a callback in KV every time it hardlinks some seqno index files to be backed up.This callback should be of the form:

void (*cb)(KVStoreID kv_id, Magma::Snapshot& snapshot, std::function<void(std::string&)> writer_fn)

and when called it should serialise into json the failover table for the vBucket and the max CAS (both read from the local index in the snapshot) and call write_fn with it.