create concise + deterministic cache key for unboundload.options

create concise + deterministic cache key for unboundload.options

Fixed issue in loader strategies where the use of the
:meth:`_orm.Load.options` method, particularly when nesting multiple calls,
would generate an overly long and more importantly non-deterministic cache
key, leading to very large cache keys which were also not allowing
efficient cache usage, both in terms of total memory used as well as number
of entries used in the cache itself.

Fixes: #6869
Change-Id: I42bd3564d55a5fb95a21d0e7eef30d50c1274da0