Convert persistence items and meta-data, etc to protobuf messages.

[jmalloc]

• queuestore.Item
• eventstore.Item
• aggregatestore.MetaData

These types are all intended for persistence and contain primitive types. If we convert them to protocol buffers messages it makes them a lot easier to persist in new provider implementations, and we can do away with the boltdb/internal/pb package which has cut-down PB representations of these types already.

[jmalloc]

Just thought I'd point out that the types used for BoltDB now are cut down from the originals, choosing not to re-persisted data that is already used as bucket-keys, for example. This doesn't necessarily stop us from doing this task, those fields could just be zeroed out to avoid duplicating them in the datastore.