Constructing with a given height

[ross-pure]

This PR adds the feature of being able to construct a replica that will start at a given height. Using this feature also fixes a problem that can occur when trying to start a replica at a later height (for instance if blocks are being stored to disk, and consensus is restarted after a crash, you would want to start from the height after the latest stored block). The problem is as follows. The usage code will usually take the form

replica := replica.New(...)

...

go replica.Run(ctx)
replica.ResetHeight(latestHeight + 1)

The call to ResetHeight will simply add a message to the message queue, to be eventually processed by the event loop in replica.Run. However, replica.Run before entering the event loop will call process.Start which will run proposal logic and hence if the process is the current proposer for height 1 round 0 it will send out a proposal, which we want to avoid but cannot, as this will always happen before the reset height message can be processed. This can cause consensus to halt in some situations. Using a constructor that allows the process to start at the desired height resolves this issue.

[jazg]

LGTM!