Fix zoekt/client issues

What does this MR do and why?

This MR fixes 2 issues with ee/lib/gitlab/search/zoekt/client.rb:

1. Memoization causes shard_id to be fixed for one client instance

irb(main):038:0> Gitlab::Search::Zoekt::Client.instance.send(:node, 2).id
=> 2
irb(main):039:0> Gitlab::Search::Zoekt::Client.instance.send(:node, 1).id
=> 2

2. URI.join doesn't work correct for our new use case. If you use non-root base_url, it returns incorrect address

[30] pry(main)> URI.join('http://gitlab-gitlab-zoekt.default.svc.cluster.local:8080/nodes/gitlab-gitlab-zoekt-0.gitlab-gitlab-zoekt.default.svc.cluster.local', '/api/search')
=> #<URI::HTTP http://gitlab-gitlab-zoekt.default.svc.cluster.local:8080/api/search>

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