vinyl: in a task use copy of index key defs to protect from alter

A vinyl space can be altered in such a way, that key definitions
of indexes are not changed, but comparators do. It is because
space format reset can make some indexed fields optional.

To be able update key definitions in place, they must not be used
in a worker thread. So lets copy key_defs for a worker, and
update index key definitions in place.

An alternative is key_defs reference counting, but there is open
questions what to do in key_defs in mems, ranges, iterators,
runs, slices. Now lets do a hotfix of a crash, and then
refactoring.

Closes #3229