Client should open only one HTTP2 connection to CAS cache server

Background

There seems to be issues to connect to the CAS cache server. Now and then clients get a timeout in connect(). While investigating that, I realized that we have multiple connections, from the client. One for each job. This could lead to poor performance and also slow down the capacity for the sever to handle accept() for new connections.

Examples of failures:

• https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/131025909
• https://gitlab.gnome.org/GNOME/gnome-build-meta/-/jobs/161511

Task description

All RPC should be made to go through one process, and multiplexed into one connection.

Acceptance Criteria

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