FR: Cache Bazel version history

During working on #438 we found out that Bazelisk spends too much time on retrieving the Bazel version history (a process that happens every time we use a relative Bazel version identifier).

The GitHub repo implementation already caches the history for one hour (https://github.com/bazelbuild/bazelisk/blob/master/httputil/httputil.go#L241). We should implement something similar for GCS.

Start by reading the existing one-hour GitHub history cache in httputil/httputil.go, then trace the corresponding GCS version-history retrieval path. The work is done when GCS history retrieval has equivalent one-hour caching and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.