vinyl: teach memory iterator to skip statements on start

With the single in-memory tree per index, read iterator will reopen
memory iterator per each range, as it already does in case of txw and
cache iterators, so we need to teach memory iterator to skip to the
statement following the last key returned by read iterator. So this
patch adds a new parameter to memory iterator, before_first, which, if
not NULL, will make it start iteration from the first statement
following the key of before_first.