Add convenient method to clean all loaded (cached) instances

[Kjuly]

Hi @marcoarment ,

Like issue mentioned in #106, when we cleaned several tables' records locally (execute sql statement directly) & try to pull data from server, these cached records will lead some problems.

e.g., we use updatedAt to judge whether need to update local record w/ the one fetched from server. As cached record will always have a same updatedAt to the one fetched from server, as a result, when cleaned local data & pull again from server, no new record will be created locally after pulling, cause we think these record exists & values are latest. Especially for the case that fetch big records one by one.

Though there's a way to load all cached instances w/ +allLoadedInstances and remove one by one w/ -removeFromCache, I think it's good to provide a convenient method to remove the whole cached array in g_instances at once. :)

[Kjuly]

Note: Updated the PR based on latest commit after merging Unique2 br.

[Kjuly]

Hi @marcoarment , plz take a review when u got time, and let me know if u've any concern about it, thx in advance.