Open a single RPC connection per backend card

Summary

Currently, one RPC connection is opened per hardware device (AMC and OptoHybrid), while the transactions are serialiazed anyway at the RPC level. It is useless, counter-production, and a waste of time and ressources.

Once !56 (merged) will be merged, using the same xHAL connection should be more or less easy to implement.

What is the expected correct behavior?

A single connection per backend board is openened for data-taking (this excludes the monitoring and AMC13 readout).