Provide a bulk method for setting metadata

[davidgcameron]

Motivation

The bulk method add_did_meta() was removed in 1.23 and now it's only possible to set single key-value pairs through set_metadata(). This will be very slow for large numbers of metadata attributes per file (70 in the case of LDMX).

Modification

Provide a bulk version of set_metadata()

[bari12]

@arisfkiaras can you please have a look at this one?
Ideally we should try to do something "performant" there, so not to fire off 1 plugin method for each key, but potentially split the bulk up per plugin and then do a bulk operation per plugin. Would that be feasible without too much change?

[davidgcameron]

Alternatively the existing methods like add_did() and add_replica() which take a metadata dictionary could be adapted to the new metadata scheme.

[bziemons]

@bari12 I have looked a bit closer at this. There is a bit of work in adding the method to the plugins, but I would be willing to take it, considering that I still have time and Python 3.7 is at the doorstep.

[bziemons]

@bari12 as David mentioned, why are the add_did and add_replica core methods not using the meta plugins for their meta parameter?

[bari12]

I think this was just forgotten. It should be, very carefully, added as well. Thanks for spotting this.
Also list_dids still points to the old method. This we carefully need to point to the new interface as well. The idea was to test the other metadata methods first before doing the big jump with list_dids.