Enable spread in satellite, use high watermark for SED

[Spaqin]

ARTIQ Pull Request

Description of Changes

This PR changes the way the lane distributor switches lanes in SED. Instead of relying on the queue being full at first and then an event being taken out, a hardcoded high watermark is implemented instead. Additionally, event spreading is enabled for satellites. For standalone or master, the watermark is set at 100% (just full), for satellite it is 75%. Exact good values would have to be determined experimentally.

Flash config option is not supported yet - would require a CSR interface, that can be added at a later moment.

Tested with Kasli 2.0 with the experiment code mentioned in #1881 that would underflow, does not underflow now.

Related Issue

Closes #1881

Type of Changes

	Type
✓	:bug: Bug fix

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Licensing

See copyright & licensing for more info. ARTIQ files that do not contain a license header are copyrighted by M-Labs Limited and are licensed under LGPLv3+.

[sbourdeauducq]

Needs update of release notes.