isisd: add a slight delay to lsp_regenerate_schedule

isisd implements a holdoff interval and will refrain from regenerating
an lsp if the difference between the current time and its last refresh
is less than the holdoff interval, but instead, it will schedule a timer
to regenerate the lsp after the holdoff interval has passed.

This implementation has one disadvantage in the case where there is a
succession of calls to lsp_regenerate_schedule. In such a case, the
first call will trigger an immediate regeneration of the lsp, while the
other calls will only schedule the regeneration timer. This leads to
cases where it takes holdoff interval time for information to propagate,
just because the information was only available e.g. at the second call
of lsp_regenerate_schedule in such a succession of calls.

By not immediately regenerating an lsp if the last generation time
is sufficiently long ago, but instead scheduling the regeneration with a
very small delay, we allow all information from such a succession of
calls to be considered.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>

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