Document and test behavior for historical queries when old ledger files are not yet available

[lynshi]

Is your feature request related to a problem? Please describe. Currently it is possible (and often desirable) to start a node from snapshot without old ledger files present. Ledger files will eventually be copied to the node's read-only ledger directory, but this allows nodes to be added faster for strengthening consensus and/or servicing requests.

It is possible for a new node to receive historical queries before the old ledger files have been copied. We believe the behavior will be that CCF will drop the query, and subsequent requests will be processed as if they were the first (i.e. as if a new historical query was just started, instead of there being one currently running).

Describe the solution you'd like As discussed with @jumaffre, the behavior in such a situation should be well documented and tested.