feature/mongo cache (#180)

* add siteID to cache Get

* indirect option setters

* add mongo cache with tests, add Key and Flusher

* lint: minor warns

* workaround for cache parallel test

* repeater in mongo cache

* missing repeater vendor

* fix nop cache

* add cache mongo benchmark

* wired mongo cache, single opts group for mongo

* disable goconst

* stop cache repeated on not found error

* use local mongo for tests in travis

Dockerfile

@@ -37,7 +37,7 @@ RUN echo "mongo=${MONGO_TEST}" >> /etc/hosts
 RUN if [ -z "$SKIP_BACKEND_TEST" ] ; then \
     if [ -f .mongo ] ; then export MONGO_TEST=$(cat .mongo) ; fi && \
     gometalinter --disable-all --deadline=300s --vendor --enable=vet --enable=vetshadow --enable=golint \
-    --enable=staticcheck --enable=ineffassign --enable=goconst --enable=errcheck --enable=unconvert \
+    --enable=staticcheck --enable=ineffassign --enable=errcheck --enable=unconvert \
     --enable=deadcode  --enable=gosimple --enable=gas --exclude=test --exclude=mock --exclude=vendor ./... ; \
     else echo "skip backend linters" ; fi