vinyl: add tx to writers list in begin_statement engine callback

Currently, we add a transaction to the list of writers when executing a
DML request, i.e. in vy_tx_set. The problem is a transaction can yield
on read before calling vy_tx_set, e.g. to check a uniqueness constraint,
which opens a time window when a transaction is not yet on the list, but
it will surely proceed to DML after it continues execution. If we need
to abort writers in this time window, we'll miss it. To prevent this,
let's add a transaction to the list of writers in vy_tx_begin_statement.

Note, after this patch, when a transaction is aborted for DDL, it may
have an empty write set - it happens if tx_manager_abort_writers is
called between vy_tx_begin_statement and vy_tx_set. Hence we have to
remove the corresponding assertion from tx_manager_abort_writers.

Needed for #4016