Set fetcher signer as worker member variables

[antiphoton]

Some SxgWorker functions take arguments of trait object Fetcher, Signer, (and HttpCache in #61). However, the implementations of these traits usually do not change during the lifetime of a worker. Hence we can move these trait objects into the SxgWorker member variables.

Changes would be like

  pub struct SxgWorker {
    ..
+   runtime: Runtime,
  }
+ pub struct Runtime {
+   signer: Mutex<dyn Signer>,
+   fetcher: Mutex<dyn Fetcher>,
+   cache: Mutex<dyn Cache>,
+ }

  impl SxgWorker {
-     pub async fn fetch_ocsp_from_ca<F: fetcher::Fetcher>(&self, fetcher: F) -> Vec<u8>;
+     pub async fn fetch_ocsp_from_ca(&self) -> Vec<u8>;
  }
}