vinyl: allow to skip certain statements on read

In the scope of #2129 we will defer insertion of certain DELETE
statements into secondary indexes until primary index compaction.
However, by the time we invoke compaction, new statements might
have been inserted into the space for the same set of keys.
If that happens, insertion of a deferred DELETE will break the
invariant which the read iterator relies upon: that for any key
older sources store older statements. To avoid that, let's add
a new per statement flag, VY_STMT_SKIP_READ, and make the read
iterator ignore statements marked with it.

Needed for #2129