folder_branch_ops: check MDCache before any GetRange calls
folder_branch_ops: check MDCache before any GetRange calls

Without this, the cache was write-only!

Also, fix flaw in range checks.  The GetRange calls would actually
return maxMDsAtATime+1 MD updates, since ranges are inclusive on both
sides.

Issue: #331

keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client