gps: source cache: enable opt-in persistent caching via DEPCACHEAGE env var

[jmank88]

What does this do / why do we need it?

This PR enables opt-in persistent source caching, via the $DEPCACHEAGE environment variable. By setting a duration (e.g. 24h), source info written to cache since <duration> ago will be accepted.

Testing lock file regeneration for cockroachdb (rm Gopkg.lock; dep ensure -no-vendor) on my machine shows speedup from around 1m30s to 10s.

This feature is opt-in only for now, so an unset $DEPCACHEAGE will not use the new cache. In the future caching may be the default, and this var would be used to override (perhaps with an additional more explicit -no-cache flag to disable per invocation).

Sources are cached under the dep cache directory as bolt-v1.db. This file can be nuked if necessary, though I have had good luck thus far.

What should your reviewer look out for in this PR?

A better env var name, or better field names in the API.

Which issue(s) does this PR fix?

Nearly #431.

[sdboyer]

Yeah, it's time for this one to go in.