index: store copy of index_def in index_read_view

To implement all iterator types for memtx tree and hash index read
views, we need to have the key definition. We can't just use the key
definition stored in the index directly, because, despite the fact that
a read view keeps a reference the index, its index definition may still
be changed by alter (when compatible changes happen).

To solve this problem, let's store a copy of the index definition in
memtx tree and hash index read views. The index definition also contains
the index name, which will be useful for exporting the read view to Lua.

Needed for https://github.com/tarantool/tarantool-ee/issues/197

NO_DOC=internal
NO_TEST=ee
NO_CHANGELOG=internal