rpc: re-use server builder per rpc interface (#6652)

This PR changes that the server builder is created once and
shared/cloned for each connection to avoid some extra overhead to
construct this for each connection (as it was before).

I don't know why I constructed a new builder for each connection because
it's not needed but shouldn't make a big difference to my understanding.

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Co-authored-by: command-bot <>
(cherry picked from commit e1add3e8)