Main/backup: Repurpose SRTO_GROUPSTABTIMEO to set the minimum timeout value

[maxsharabayko]

Consider repurposing SRTO_GROUPSTABTIMEO to provide the minimum timeout value.

PR #1775 introduces runtime link stability timeout (RLST), making SRTO_GROUPSTABTIMEO unused.
However, both runtime stability timeout (RLST) and initial link stability timeout (ILST) have a constant minimum allowed value of 60ms. Based on experimental data this value is considered an optimal one, but in some cases, it might be useful to allow specifying it via a socket option.
SRTO_GROUPSTABTIMEO is a good candidate for it.

Minimum Link Stability Timeout = 60 ms.
ILST = max(Minimum Link Stability Timeout, SRT Latency);
RLST = min(max(Minimum Link Stability Timeout, 2RTT + 4RTTVar), SRT Latency)

TODO

• Rename SRTO_GROUPSTABTIMEO to SRTO_MINLINKSTABTIMEO and use it as the minimum link stability timeout.