add reference id for idempotent refence counter operations

[xiegeo]

WIP (not stable until one stable server-side and client-side implementation)

The basic design for the protocol, server and client sides to be implemented.

When operating on a reference-counted block store, exact once operation is required to keep the count correct. Although exact once operation is trivial when down locally, it is impossible to guaranty over an unreliable network.

This is worked around by adding a nonce (reference id) to each add operation that can only be removed by calling remove on the same nonce. Not only does this make the operations idempotent (we can safely retry the same operation), it can also prevent users from removing other's user's adds.

For example, the s3x client can implement this nonce by hash(client private key | server public key | bucket name | object name). The use of the private key prevents attackers from guessing the hash (and server key prevents cross-server attacks), while bucket and object names allow repeated counts on the same content. Everything is also generated statically so nothing extra needs to be saved for deletion.

Problems

• The java code generation plugin failed

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Codecov Report

Merging #100 (7ee9da6) into master (e4ee4ab) will decrease coverage by 2.48%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
- Coverage   72.60%   70.12%   -2.47%     
==========================================
  Files           9        9              
  Lines         405      338      -67     
==========================================
- Hits          294      237      -57     
+ Misses         84       74      -10     
  Partials       27       27              

Impacted Files	Coverage Δ
go/rep_server_source.go	80.00% <0.00%> (-4.61%)	:arrow_down:
go/rep_client.go	53.34% <0.00%> (-3.58%)	:arrow_down:
go/rep_addr_info.go	85.00% <0.00%> (-2.75%)	:arrow_down:
go/rep_subscription.go	81.42% <0.00%> (-2.28%)	:arrow_down:
go/rep_replication.go	87.50% <0.00%> (-1.78%)	:arrow_down:
go/rep_proto.go	100.00% <0.00%> (ø)
go/dag_service.go	0.00% <0.00%> (ø)
go/rep_sort_bytes.go	100.00% <0.00%> (ø)
go/xtestutils/testutils.go	100.00% <0.00%> (ø)

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