[Go] allocation costs on latency (Go 1.15)

Reference:

https://groups.google.com/forum/#!msg/golang-nuts/v_kYqKJKyRc/TGUJ_zJrAAAJ

Go doesn't sweep an arbitrary number of spans. 

In the current implementation, on the slow path, it looks through a
limited number of spans to find one that has space, and then it sweeps
that one and uses it.

It checks up to 100 spans looking for one to sweep; if it doesn't find any in the first 100 that it checks, it allocates a new span. So, there is an upper bound on allocation time.