vinyl: do not store meta in secondary index runs

Currenlty, tuple meta is only needed for storing statement flags in run
files. In the scope of #2129 two statement flags will be introduced,
VY_STMT_SKIP_READ and VY_STMT_DEFERRED_DELETE. None of them makes any
sense for secondary indexes. If we encode meta for secondary index
statements, we will have to either clear the flags on the upper level
(e.g. in the write iterator) or filter them out before encoding a
statement. Alternatively, we can skip encoding meta for secondary index
statements altogether, and this is what this patch does, because it's
the simplest and clearest method for now. If tuple meta is ever used for
storing anything else besides statement flags or a new statement flag
appears that may be used with secondary index statements, we will
recover the code and mask out those flags for secondary indexes.