Update core namespace endpoint to use client pool

[Callisto13]

Short description
Following on from #1600 and based on #1556, this is to update the endpoint in core/server/namespace to:

• Pull a list of clients from the ctx
• Loop over those and query each

Acceptance criteria
Everything which currently works should still work

[jpellizzari]

This is sort of being worked on in #1725

[Callisto13]

I don't think it is exactly. Looks like it is using a single client. But is worth noting that this one should probably wait for #1780 to be merged, just to avoid complications

edit: oh wait I got confused about which ticket this was (either way, it may be easier for you to wait for 1780 before you do 1725)

[yitsushi]

The scope of this ticket changes with my PR on namespace cache (#1780) because I replaced the namespace grpc endpoints to use the cache instead. So it's more like:

update cache/namespace to use client pool, pull and store data respectively.