Implenent SL LinksetData methods

[renevega]

Tracking the ⁠off-topic comments to get SL's LinksetData into OpenSim. Here is the arch-spec:
https://wiki.secondlife.com/wiki/Category:LSL_LinksetData
Second Life Wiki

Mike Chase — Today at 4:52 PM
It is a key value store and I'd probably use json for the representation internally. But the api is fetching values given a key so pretty simple to do with json and string in the database

Balpien Hammerer — Today at 4:52 PM
Yes, the architectural spec, which is K/V oriented, should be followed, though not
necessarily a key/value implementation in the sense of a database.

The fact there is a limit to the size strongly suggestions a serialized json implementation, just like High Fidelity did.
And, this makes it clear its serialized in the root prim: ..."It is possible for data to be rolled back to a previous state if the datastore is stored in an object that is restored via a simulator rollback, or in an attachment that is not properly saved back to the server on logout...."

Mike Chase — Today at 5:56 PM
Just adding that there is a "JsonStore" implementation in OptionalModules that could probably be adapted.

[Misterblue]

The underlying stored data could be a JSON string and additional functions to just store and fetch the JSON string could be added. The llLinkset* functions would be just operations on the JSON string. There could be a JSONPath operation on the JSON string also.

[renevega]

Am implementing this. The prims table will have a mediumtext field, LinkSetData default NULL. The linkset data is stored in the root prim (linkNumber 0) as a json string. This data is decanted into an ordered associative list on SOG instancing and reserialized on SOG persist. All other operations act on the in-memory list.

[zontreck]

See #64