[fastx distsys] Refactor Sequence number

[patrickkuo]

In the current design, each client have one sequence number counter next_sequence_number to keep track of the client's certificate history, however this become a problem when we try to retrieve certificate from authority because we don't record the association between object id and sequence number, we don't know which authority to query from for a particular sequence number.

The current work around is looping through each authorities until we have a hit, but this is inefficient.

[patrickkuo]

I think the solution is to have a sequence number per object, so each shard will have its own sequence number.

Alternatively, we can simply record the relationship by keeping a Map of ObjectID to Sequence number in ClientState

[sblackshear]

I think a sequence number per object is indeed the way to go (similar to what's being done on the authority side in #39).