Semantics for query start and end records

[jfunston]

Certain types of scans are difficult or awkward to represent with the current query semantics. For example, prefix scans need to be specified by leaving fields unspecified and having the start and end records be the same, which doesn't indicate intent well. Another problem is being able to scan to the end of a particular field and include all records.

We should add these two methods to SKVRecord: serializeStopLow() and serializeStopHigh(). These methods will add the correct bytes to serialized key in the same way it is done now automatically. Calling these methods will prevent further serialization. The query will ensure that either the records are fully specified or one of these methods has been called.

We can also remove the user-specified scan direction and automatically determine it from the start and end records.