v0.1.0

initial release of the DAG store, supporting:

• registration of shards, both with eager and lazy initialization
• mounts system for data location referencing
• "upgrader" mount that mirrors a remote/non-random-access/non-seekable shard into a local, seekable, random-access shard
• acquisition of shards at any time, regardless of state of the shard
• shard accessor abstraction to enable different access patterns to shard data; right now supporting Blockstore()
• release of shards
• failure notifications and management
• failure recovery through explicit call to RecoverShard
• event tracing
• refcounting and GC
• more!