Remove root span and max_nested_level

[bonnefoa]

Root span in the trace_context was used to deal with the extended protocol + transaction block issue: The parse of the next statement happens while the previous statement is ongoing since the portal is only dropped during the bind command.
We can replace it by a specific next_active_span to store span's information when we hit this case and restore it when needed.
We also remove max_nested_level: we can track whether pg_tracing memory context is allocated by checking pg_tracing_mem_ctx->isReset, making max_nested_level redundant.