* Go: Back datas.unwrittenPutCache with a LevelDB
httpBatchStore was caching as-yet-unwritten Chunks both in memory and
on disk. To avoid this, the in-memory cache is now backed by a
LevelDB, which handles spilling to disk when it needs to. When it's
time to send Chunks to the server, the cache is enumerated in
insert-order so that the payload of the write request is properly
structured.
Fixes#1348
Using ChunkStore.PutMany() means that the DataStore server code
can detect when the ChunkStore it's writing to can't handle
the amount of data being pushed. This patch reports that
status back across the wire to the client that's attempting
to write a Value graph. Due to Issue #1259, the only thing the
client can currently do is retry the entire batch, but we hope
to do better in the future.
This patch is unfortunately large, but it seemed necessary to make all
these changes at once to transition away from having an HTTP
ChunkStore that could allow for invalid state in the DB. Now, we have
a RemoteDataStoreClient that allows for reading and writing of Values,
and performs validation on the server side before persisting chunks.
The semantics of DataStore are that written values can be read back
out immediately, but are not guaranteed to be persistent until after
Commit() The semantics are now that Put() blocks until the Chunk is
persisted, and the new PutMany() can be used to write a number of
Chunks all at once.
From a command-line tool point of view, -h and -h-auth still work as
expected.
