fix: performance issue with fiber::csw

Getting fiber.info() is a rather expensive operation. By default it implicitly collects all fibers backtraces. It appeared to be a bottleneck for one of the tests.

Fortunately, Tarantool 2.10 provides a better API fiber.self():csw().

For older versions we use {bt = false} option as a fallback.

See also:

• https://www.tarantool.io/en/doc/latest/release/2.10.0/#fiber

Close #133 (closed)