kvs: define maximum size

[chu11]

Highly related but somewhat different to #6264 and #6265, we should define what the maximum value in the KVS is. It is nominally 2G - 1 b/c that is the largest value an int can hold and we use an int in treeobj_create_val().

But that need not be the case, it could be something else. We could also support unsigned int or size_t for the length parameter in treeobj_create_val().

An update to RFC11 should probably be done too.