vinyl: fix read iterator skips source after reading cache

If a source is used on a read iteration (i.e. the key at which it is
positioned is the next best match or, in terms of the read iterator
implementation, its front_id matches the read iterator front_id), its
history is cleaned up, see vy_read_iterator_apply_history(). This breaks
the logic behind vy_read_src_is_behind(), which assumes that the history
always points to the last used key. As a result, a source may be
mistakenly skipped, as illustrated below:

  Fiber 1                               Fiber 2
  -------                               -------
  1. Opens read iterator.
  2. Advances it to the next key.
     The returned key was read from
     a mem or run (not from cache).
     The source's history is emptied.
                                        Adds a chain containing
                                        the key read by fiber 1
                                        to the cache.
  3. Continues iteration, reads
     next few keys from the cache
     until the chain ends. The source
     used at step 2 is skipped.
  4. Calls vy_read_src_is_behind()
     on the source used at step 2 and
     skipped at step 3. It returns
     false, because its history is
     empty, thus skipping keys stored
     in it.

Fix this bug by moving the code that checks whether a source iterator
needs to be advanced from vy_read_src_is_behind() to source iterator
'skip' method, because there we always know the last key returned by
the iterator.

Basically, this returns the code we had before commit b4d57284
("vinyl: consolidate skip optimization checks in read iterator").

Closes #3477