Cleanup the code in internal/files/leveldb.go a bit.

This commit introduces the following, cosmetic changes:

- All callers of 'ldbGenericReplace' specify a non-nil 'deleteFn' argument.
  Thus, remove the checks for 'deleteFn' being nil.

- Before this change, 'ldbInsert' took a 'FileInfo' argument and a
  separate argument for the file name, but all callers passed in the
  value of FileInfo.Name as the file name.  Simplify this, by not not
  passing in the file name as a separate argument.  This makes the
  function signature of 'ldbInsert' more similar to 'ldbUpdate'.

- doc fixes
Merge pull request #829 from seehuhn/cleanups

Cleanup the code in internal/files/leveldb.go a bit.