This commit replaces the client in provisioners and webhooks with an
interface. Then it implements the interface using the new poolhttp
package. This package implements the HTTPClient interface but it is
backed by a sync.Pool, this improves memory, allowing the GC to clean
more memory. It also removes the timer in the keystore to avoid
having extra goroutines if a provisioner goes away. This commit avoids
creating the templates func multiple times, reducing some memory in the
heap.
This commit avoids an error starting the CA if the `http.DefaultTransport`
is not an `*http.Transport`. If the DefaultTransport is overwritten, the
newHTTPClient method will return a simple *http.Client. With an
*http.Transport, it will return a client that trusts the system
certificate pool and the CA roots.
