Store entry responses.

Store responses fetched by async log client, and use them when available. Since we aren't using unsafe pickle I've set use_stored_entries flag to true by default. 
However, storing one file per EntryResponse might not be the best aproach.
You can simply merge the two flags into one, "persist_entries", or "use_entries_cache" which will persist the entries to disk unless they're already there, in which case they'll be simply read.