http. RoundTripper is not re-used

[pavelmemory]

For the bitbucketserver and github vcsclient(s) the http.RoundTripper is not re-used, but created for each new request if client.vcsInfo.Token is provided.
This can cause an issue because connections acquire file descriptors and won't be released immediately after http.Client garbage collection, please see https://golang.org/src/net/http/transport.go#L68

[eyalbe4]

@pavelmemory, Thanks for reporting this issue. We'd be happy to accept a fix for this issue through a pull request.