gid-ex: Enhance consideration input for epoch length in Gid-reusing GMs

[chrysn]

This is a proposal from my upcoming review:

C, "Thus, the expected highest rate for addition/removal of group
members and consequent group rekeying should be taken into account for
a proper dimensioning of the Group Epoch size." When the GM is capable
of reassigning GIDs, is this really a concern? I'd expect that if
triggers to rekeyings happen with a frequency in the order of
magnitude of the rekeying time, then the rekeyings themselves would be
delayed rather than churning through parallel rekeyings. (See also the
comment about "undue delay" -- and if that wording is not accepted,
the GM can still make the "evict device" operation take some time). So
rekeyings would always be roughly ordered, and even a nibble of group
epoch values would by far suffice.