refactor: improve tracing span usage

[prestwich]

Makes a few span usage changes

• adds verbose event for param parsing failures in Level::TRACE
• adds root spans for request and batch request handling in router
• removes permanently-open spans for tasks
• moves future instantiation after permit acquisition to delay resource allocation
• adds span to BatchFuture so that all RouteFutures are inside the batch future
• add spans that record the params before decoding, and before deserialization

[Evalir]

no nits