Tests for the local state query server.

The local state query protocol allows clients such as wallets to query the state of the ledger at any point within k blocks from the tip. The test for this is quite minimal at present: it prepopulates a ledger DB with a bunch of blocks, and then verifies that requesting the ledger tip corresponding to the these blocks gives the right answers, and that asking for blocks not on the chain results in the right error message.

Note that the query protocol is abstract in the ledger, and the query language we offer (the kinds of queries that can be asked) of course depends on the ledger. The tests use a mock ledger for this purpose.