Make date range start fully optional when querying

[nblumhardt]

Since query ranges can now be specified using @Timestamp constraints, the rangeStartUtc argument to QueryAsync() and QueryCsvAsync() should be optional.

E.g. it's fine to issue:

var csv = await connection.Data.QueryCsvAsync("select count(*) from stream where @Timestamp > Now() - 1h");

Binary-breaking change. Version number not revised as per the policy of tracking corresponding Seq server versions.