vy_index->space is zeroed on index drop, so we have to be extremely
careful about how to use it, e.g. we can't dereference it in a dump
task, while currently we do, which can result in a crash. Actually,
vy_index->space is only used for retrieving the primary key, so having
a pointer to the whole space in a vy_index is kinda superfluous - a
primary key would be enough. That being said, this patch replaces the
space field with the pk field in the vy_index struct. vy_index->pk is
referenced by each secondary index so it's safe to use it wherever we
want. Also, add a test dropping a space in the middle of dump - w/o
this patch it leads to a crash.