vtgate routing cleanup: remove routeOptions

[sougou]

I previously added an experimental feature that allowed the routing schema to provide multiple viable targets for a table (if it was materialized in other places). This would allow vtgate to choose the best option depending on the query.

The feature was implemented using the "routing options" approach. It was not a good design because it wasn't going to play well with future optimizer approaches. The main difference is that it optimizers like to try multiple options top-down, whereas routeOptions was implemented using something low level and mostly opaque. Additionally, it distorted the design of the symbol table by moving some of those elements into the routing Option.

This change removes all those complexities.

• RoutingRules has been changed to allow at most one target per table.
• routingOptions have been removed, and the metadata of the only alternative has been merged back into either the route or the symtab.

Along the way, I found some subtle bugs in the code that evaluates mergeability of routes, which I've fixed. I've also filed #6529 to make sure SelectNone is handled correctly.

[sougou]

This is mostly an undo of this PR #4833

[harshit-gangal]

I went ahead and resolved merge conflict.
All checks passed. so merging.