Report all refreshCheck() outcomes and entry gist

[rousskov]

All other refreshCheck() return statements already have a debugs().
Reporting StoreEntry gist helps find the right calls in busy proxy logs.

[rousskov]

I spent 10+ extra minutes trying to confirm refreshCheck() outcome for a particular test and decided to save that time by reporting the last two "silent" return statements... At least on of these two outcomes can be correctly identified using existing earlier debugs() statements in refreshStaleness(), but those statements are in another function and are not necessarily the last debugs() in the log before refreshCheck() exits, making analysis more difficult than it should be.

P.S. This review annotates this PR without requesting any changes.

[rousskov]

Some existing debugs() in this function already use this (correct) minimalist style to report outcomes.

Some debugs() also add "YES/NO prefixes that are rather misleading for a non-boolean function that is not called something like isFresh(). Some debugs() also try to explain why a particular outcome was reached by describing the last condition checked, but such reporting can easily mislead because all but the very first outcome depend on multiple preconditions. For the two return statements reported in this PR, just letting the developer know which exit path got triggered is the best strategy IMO.