alter: do not require index rebuild to clear uniqueness

Obviously, there's no point in rebuilding an index if all we do is
relaxing the uniqueness property. This will also allow us to clear
the uniqueness flag for vinyl indexes, which do not support rebuild.

Note, a memtx tree index stores a pointer to either cmp_def or key_def
depending on whether the index is unique. Hence to clear the uniqueness
flag without rebuilding the index, we need to update this pointer. To do
that, we add a new index virtual method, update_def.

Closes #2449