This commit makes the rarely used `raftpb.Message.Snapshot` field nullable.
In doing so, it reduces the memory size of a `raftpb.Message` message from
264 bytes to 128 bytes — a 52% reduction in size.
While this commit does not change the protobuf encoding, it does change
how that encoding is used. `(gogoproto.nullable) = false` instruct the
generated proto marshaling logic to always encode a value for the field,
even if that value is empty. `(gogoproto.nullable) = true` instructs the
generated proto marshaling logic to omit an encoded value for the field
if the field is nil.
This raises compatibility concerns in both directions. Messages encoded
by new binary versions without a `Snapshot` field will be decoded as an
empty field by old binary versions. In other words, old binary versions
can't tell the difference. However, messages encoded by old binary versions
with an empty Snapshot field will be decoded as a non-nil, empty field by
new binary versions. As a result, new binary versions need to be prepared
to handle such messages.
While Message.Snapshot is not intentionally part of the external interface
of this library, it was possible for users of the library to access it and
manipulate it. As such, this change may be considered a breaking change.
Signed-off-by: Nathan VanBenschoten <nvanbenschoten@gmail.com>
Usually the consistent_index should be greater than the index of the
latest snapshot with suffix .snap. But for the snapshot coming from the
leader, the consistent_index should be equal to the snapshot index.
