Limit manifest's change set size (#1119)
Limit manifest's change set size (#1119)

This PR limits the amount of memory we allocated for reading
the manifest changes set's size. When a manifest file is corrupted,
in the worst case we might end up allocating more than 4GB.

This PR ensures we don't over-allocate the byte slice.
Fixes https://github.com/dgraph-io/badger/issues/490
dgraph-io/badger,dgraph-io/badger