This changes so that all commit struct types have a meta field
(which might be an empty struct).
Increment the serialization version since the old data does not
necessarily have the meta field.
Fixes ##2112
Previously when we had an ordered (set/map) prolly tree containing
non-ordered values (blobs/refs/etc), we'd put the ref of the largest
value in each meta node, complete with its full type info and height.
This is wasteful, all we really need is the hash of the largest item for
searching the tree. In lieu of encoding just the hash - which isn't a
value - this patch creates a fake Ref<Boolean> with height 0.
ChunkStores provide a Version() method so that anyone directly
using a ChunkStore (e.g. BatchStoreAdaptor) can retrieve and
check the version of the underlying store.
remoteDatabaseServer checks the version of the ChunkStore it's
backed by at startup, and then provides that version as an HTTP
header to all clients. In Go, httpBatchStore checks this header
anytime it gets a response from the server and bails if there's
version skew.
In Go, the responsibility for checking whether the running code and
the data being accessed lies with the BatchStore layer. In JS, there
is code in fetch.js that checks the header mentioned above.
Towards #1561
Aaron Boodman