sync.Pool should be passed by reference

[sodaRyCN]

The sync.Pool used in filter/proxy/pool.go should be passed by reference instead of by value.
use go vet command go vet pkg/filter/proxy/*.go , then get the warnings.

note: objects in sync.Pool that are not explicitly cleaned up before call Put may cause dirty data to be obtained the next time call Get. I'm guessing httpStatResultPool might be the case, but I need experiments to prove it. EG:When the first request succeeds, httpstat.Result's some properties will be set, and when the second request fails (failed to resolve the domain name, failed to establish a tcp link and etc.), some properties (NameLookup\TLSHandshake\serverDone) may use the last one. value，that may be leads to a bias in the statistics

[localvar]

Thanks, I think this PR fixes an important bug.