fix: trasaction already started error

Remove redundant transaction start from the open telemetry's spans in the code. There is no sense in it at the moment (seems like it was added while prototyping tarantool space as a storage for the per-fiber traces... but we gave this idea up and switched to the custom Rust implementation). But if we start a transaction manually and call sbroad execute() method, we end up with transaction already started error. So, remove the transaction start wrappers and make nested transactions work.

Fixes #335 (closed)

We can't implement integration tests at the moment as for performance reasons internal tracer collects only 1% of all running queries. We need to implement #336 first to solve this problem correctly.