Fix clone redirect revisit checks

[StantonMatt]

Summary

• fix cloned collectors following redirects with the parent collector's redirect checks
• store the request-owning collector in the request context before dispatch
• keep the shared HTTP backend while routing CheckRedirect through the collector that created the request

Fixes #875.

Verification

go test . -run 'TestCollector(CloneRedirectUsesCloneRevisitSetting|URLRevisit|Depth)|TestCollectorRedirect' -count=1 -v
go test ./... -count=1
gofmt -l colly.go colly_test.go
git diff --check origin/master...HEAD

I also checked the regression test against current master before the fix; it failed with AlreadyVisitedError when the clone had AllowURLRevisit = true and the parent did not.