abandon generic query frag for common structures
Instead of using the QueryFrag struct, which was too generic to be
generally useful, we have moved to a BusEventFilter type, which
contains things we may filter on when we're searching for bus events.
At the moment it just contains names, and start/stop times.
Then in each function we can accept this filter struct and convert
it to fit the query.
We also support general modifiers, and currently have one implemented:
the LimitOffsetModifier. This adds a LIMIT and OFFSET clause to any
statement. these are all applied at the end and receive a stringbuilder
which may prevent certain operations from being structured.
We need to work on this one more, potentially abandoning.
