[#493] Refactor serving of prometheus and pprof services

Rename `util/profiler` package to `httputil` and refactor it:

  * simplify utility HTTP server;

  * make more generic server's parameters in order to remove `viper.Viper`
    dependency;

  * use single constructor for creating the pprof and prometheus servers;

  * replace `enabled` config value with empty-check of the network address.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/util/http/pprof.go

@@ -0,0 +1,16 @@
+package httputil
+
+import (
+	"net/http"
+	"net/http/pprof"
+)
+
+// initializes pprof package in order to
+// register Prometheus handlers on http.DefaultServeMux.
+var _ = pprof.Handler("")
+
+// Handler returns http.Handler for the
+// Prometheus metrics collector.
+func Handler() http.Handler {
+	return http.DefaultServeMux
+}