gps: source cache: interface tweaks and implementation optimizations

[jmank88]

What does this do / why do we need it?

Changes a singleSourceCache method's signature from storeVersionMap(versionList []PairedVersion, flush bool) to setVersionMap(versionList []PairedVersion), because all current usages flush and it simplifies the implementations (particularly for the persistent cache, since timestamps/TTLs come into play).

There are also some small related optimizations sprinkled in.

What should your reviewer look out for in this PR?

Reasons to leave the flush parameter as is.

Do you need help or clarification on anything?

Nope.

Which issue(s) does this PR fix?

Related to #431 - Preparation for persistent cache.

[sdboyer]

hmmm...i have a vague recollection that i implemented it in the same way this PR changes it to initially, but then switched to the store-and-flush model later. unfortunately, the git blame doesn't have a history on that; even if my recollection is correct, i don't have any hints as to what made me change my mind.

does this make things significantly easier for what you're doing? if so, then i'm OK to bring it in. if not, i'd prefer to keep it as-is, on the assumption that whatever informed my original choice may come back up later, and that we may be happy to have less refactoring, or even rearchitecting, to do then.

[jmank88]

does this make things significantly easier for what you're doing?

Yes, both because it's a simpler model to timestamp the whole version set (e.g. don't have to worry about juggling multiple incomplete sets or individual values with differing timestamps), and because it's already implemented.

[sdboyer]

ok, good enough 😄