box: introduce key_def->is_multikey flag

Previously only key definitions that have JSON paths were able
to define multikey index. We used to check multikey_path != NULL
test to determine whether given key definition is multikey.
In further patches with functional indexes this rule becomes
outdated. Functional index extracted key definition may be
multikey, but has no JSON paths.
So an explicit is_multikey flag was introduced.

Needed for #1260